Synthesis and Diels-Alder reactivity of sulfinyl homo-and hetero-dienes obtained via enantio-pure sulfenic acids

This account is intended to provide a brief survey on our contribution in the field of enantiopure sulfinyl homo and hetero dienes which we are able to obtain via enantiopure sulfenic acids. The chemistry of these sulfinyl-dienes was mainly developed taking into account the practical importance of stereoselective homo and hetero Diels-Alder cycloadditions, able to supply useful intermediates in the synthesis of target molecules


Introduction
Since the pioneering work in 1983, 1 enantiopure vinyl sulfoxides have been commonly employed as dienophiles in Diels-Alder (DA) reactions.Their use often results in high levels of regio-and stereo-selectivity. 2 Recent years have witnessed an almost explosive development in the syntheses of conjugated diene sulfoxides and a growing interest in their use in stereoselective DA reactions. 3The results obtained have emphasized the sulfinyl group's efficiency in controlling π-facial diastereoselectivity and the synthetic potential of conjugated sulfinyl dienes in the field of natural product chemistry. 4his report is mainly devoted to a survey of our contributions to the synthesis of enantiopure sulfinyl homo-and hetero-dienes, and investigations of their reactivity in DA cycloadditions.The first part of the review deals with a description of the general methodology for the preparation of enantiopure diene sulfoxides, based on the site-selective addition of sulfenic acids to suitable acceptors.The second part illustrates the high degree of stereocontrol observed in cycloadditions of cyclic-, acyclic-, homo-, and hetero-dienophiles with sulfinyl homo-and inverse-electron-demanding DA reactions, so giving a straightforward access to functionalized and enantio-pure pyranoid systems. 8The addition of 1 to the triple bond of dihydroethynylnaphthalenes 4 gave two dihydrovinylnaphthalenes 9 and 10, epimeric at sulfur, in good total yield and in different amounts.The DA reactions of such dienes with the appropriate dienophiles give a straightforward approach to estrone-like compounds.
Camphorsulfonic-and mandelic acids, which are readily available members of the chiral pool, provided the precursors for the hydroxythiols 11d,f,g which were chosen because the contiguity and consequent intramolecular hydrogen bonding between hydroxy and sulfoxide functions in their derivatives 1, 5-10, 13 facilitate the chromatographic separation of diastereoisomers and enhance the stereoselection in concerted processes.2-Hydroxy-1-phenylethanethiol, 11f, and 2-hydroxy-2-phenylethanethiol, 11g, were exploited as chiral control elements since the chiral auxiliaries derived from these vicinal hydroxythiols may be removed by phenyloxiran formation. 9Moreover, we decided to compare the behavior of the structural isomers 11f and 11g, in order to evaluate whether the position of the asymmetric carbon with respect to the sulfur atom could have any effect on the asymmetric induction during the thermolysis of sulfoxides 13 (Scheme 1).The commercially available [(1S)-endo]-(-)-borneol was easily transformed into the corresponding thiol 11e which represented a suitable precursor of sulfinyl-dienes 5-10e possessing the remarkable characteristics of the camphor skeleton but having no possibilities of intramolecular hydrogen bonding.As a matter of fact, an intramolecular hydrogen bonding requires the presence of a hydroxy group in a suitable position with respect to the sulfoxide oxygen atom.This can be a limitation both in the choice of starting products from the chiral pool and the chemical behavior of the synthesized diene systems, i.e., some unexpected and undesired reactions of the hydroxy function 10 may occur during subsequent chemical transformations.Our final purpose was the assessment of the structural characteristics required by chiral precursors to be effective in our synthesis of enantiopure sulfinyl dienes, in order to provide a larger number of terpene derivatives in the pool of suitable starting products.The obtention of the epimeric dienes 5, 6, 9, 10 and ketones 7, 8 in good yields, and their almost general easy separation by chromatography, represent favorable features for synthetic application.

Diels-Alder reactions of enantiopure alkyl sulfinyl homo-and hetero-dienes. The Effects of Lewis acids
Most of our investigations have involved the readily accessible dienes 5b,c and 6b,c which proved to be reactive DA participants with both cyclic and acyclic electron-deficient carbodienophiles.They undergo cycloadditions at room temperature or below, whereas the diene 5ad reacted much more sluggishly with methyl acrylate or N-phenylmaleimide (NPM)-an expected consequence of its (Z)-configuration. 11ome relevant data regarding the DA reactions of dienes 5 and 6 with common acyclic dienophiles such as methyl acrylate, dimethyl maleate and fumarate (Scheme 2) are given in Table 1.In almost all the cases cycloadditions occurred with complete regioselectivity but the uncatalyzed DA reactions were not particularly stereoselective, showing in some experiments low endo/exo and facial diastereoselectivities.

Scheme 2
The effect of Lewis acid catalysis was investigated in some detail with diene 5bd (entries 3-13 in Table 1).All the catalysts investigated, except BF 3 .Et 2 O, served markedly to increase the endo/exo ratio, and the use of LiClO 4 or ZnCl 2 in CH 2 Cl 2 led also to high diastereofacial selectivity, apart from the special case of dimethyl fumarate.The best catalyst was LiClO 4 , whose use as a suspension in CH 2 Cl 2 gave only the endo isomers 15, 16, 23, 24, 27, 28 in high yield and very high facial diastereoselectivity.This was the first illustration that LiClO 4 suspended in CH 2 Cl 2 catalyzed DA reactions, 12 and that it did so highly stereoselectively.Reetz and Fox 13 have shown that Mukaiyama aldol reactions and conjugate additions were more efficiently catalyzed in this way than by 5 M LiClO 4 in Et 2 O, conditions which were previously advocated for the dramatic acceleration of DA reactions. 14 rationalization of the stereochemical features observed in these cycloadditions has been proposed, and is discussed here for the DA reaction of diene 5bd with methyl acrylate (Scheme 2).
The predominance of 15 among the products of the uncatalyzed cycloaddition of 5bd may be interpreted on the basis of Re face approach of methyl acrylate to the diene in its (h) conformation, taking into account the relative stabilities of the transition states originating from the endo approach of the dienophile to different conformations of the diene around the S--C-3 bond.The isolation from the same reaction mixture of a certain percentage of the diastereomer 16 may be explained by considering the Siface approach of the dienophile to the diene in the less favored (i) conformation.The great improvement of diastereoselectivity and increased reaction rate observed in the LiClO 4 catalyzed DA reaction of 5bd suggest the approach of methyl acrylate to the Re face of the diene in its (j) conformation, with the catalyst coordinating the sulfinyl oxygen of the diene and the carbonyl oxygen of the dienophile, as depicted in Figure 1.

Figure 1
The failed increase of facial diastereoselectivity in the LiClO 4 catalyzed cycloaddition of (R S )-1-{1-[(1S)-isoborneol-10-sulfinyl]vinyl}cyclohexene (5cd) with methyl acrylate (Table 1, entry 15) was unexpected on the basis of the results obtained in the reaction of diene 5bd with the same dienophile and in the presence of the same catalyst.However, the observed coordination via Li + of both the diene and dienophile requires a suitable diene conformation in the transition state where the sulfinyl oxygen is directed towards the dienophile; this situation could be too sterically demanding and thus unfavored when 5cd and methyl acrylate are involved in a DA reaction.
Literature data on the cycloaddition of dimethyl maleate with (E)-1-methoxybuta-1,3-diene report the formation of endo/exo products in an approximately 70:30 ratio. 15Our observation of complete endo diastereoselectivity in the reaction between 5bd and maleate (entry 13 in Table 1) indicates the remarkable control exerted by a sulfinyl group on the endo selectivity when the dienophile is suitably substituted.

Scheme 3
An additional confirmation of the observation that the asymmetric induction in these DA reactions of sulfinyl dienes is overwhelmingly influenced by the sulfur configuration can be inferred from a careful inspection of Table 1, where the results of DA reactions of dienes 5bd, and 6be-g (entries 6, 18, 20, 23) with methyl acrylate in the presence of LiClO 4 are reported.While the structural features of the alkyl residues directly linked to the sulfur function in enantiopure alkyl sulfinyl dienes play a relevant role in their synthesis based on sulfenic acid / enyne addition, the stereogenic nature of these alkyl skeletons has no significant effect on the stereocontrol observed in the DA reaction of such sulfinyl dienes.
Electron-deficient cyclic dienophiles, such as maleimides and maleic anhydride, have been used widely in cycloadditions with enantiomerically pure 1-and 2-sulfinyl-dienes, and complete endo-, and very high facial-diastereoselectivities were generally observed.Dienes 5 and 6 were reacted with maleimide and NPM, and the results of these uncatalyzed and catalyzed cycloadditions are summarized in Table 2.The nearly exclusive formation of 38, 42, and 50 (Scheme 4, entries 3, 8, 11 in Table 2) indicate the great control exerted by a sulfinyl group on the π-facial selectivity: the cyclic dienophiles approach dienes on their Re faces, which are the more nucleophilic sides, opposite to the sulfinyl oxygen, with the dienes adopting the less sterically hindered conformation along the C-S bond.
Although the results on both the reactivity and stereoselection of dienes 5 and 6 with cyclic electron-deficient dienophiles are impressive, we found of interest the progression of our investigation into the effect of Lewis acid catalysis.The cycloadditions of our dienes with maleimides occurred with reduced reaction times, and sometimes complete endo/exo diastereoselectivity in favor of the endoisomer (compare entry 10 with 11, and 14 with 15 in Table 2) if performed in the presence of LiClO 4 , which was chosen for its good ability in enhancing facial diastereoselectivity.The reported lack of influence, or decrease in facial diastereoselectivity (compare entry 2 with 3, 6 with 8, and 10 with 11 in Table 2) induced us to investigate the cycloadditions of diene 5bd with NPM in the presence of different Lewis acids: all of these led to a diminished facial selectivity, which turned over to favor the diastereoisomer 51 when Eu(fod) 3 was the catalyst (entry 5 in Table 2).b The minor endo adduct was not isolated from the reaction mixture.
The stereochemical outcome of these catalyzed reactions can be interpreted by considering the endoapproach of the dienophile once again on the Re face of the diene, which adopts the conformation (j) where the catalyst is able to co-ordinate both the carbonyl oxygen of the dienophile and the sulfinyl oxygen of the diene.However the dienophile's approach to the Si face of the diene in its (k) conformation also becomes effective owing to the catalyst participation and steric requirements of a conformationally biased dienophile such as NPM: the (k) conformation appears electronically less favored than (j), but this adverse characteristic is counterbalanced in part by a reduced steric congestion in the transition state.If this is true, the size of the Lewis acid should influence the relative stabilities of the transition states originating from NPM's endo approach to conformations (j) or (k): in fact, the approach to the Si face of the diene 5bd in its conformation (k) becomes the favorite one when the bulky Eu(fod) 3 catalyzes the cycloaddition (entry 5 in Table 2).NPM was also chosen as the dienophile in DA cycloaddition with dienes 9 and 10, since the reaction would lead to enantiopure cycloadducts showing the 16-azasteroid skeleton.On the basis of present knowledge, 16-azasteroids do not exist in nature but they have been synthesized in racemic form and biologically tested. 16The conditions and results of the DA reactions performed are reported in Table 2.The diastereomeric cycloadducts 52-61 (Scheme 5) were obtained in very good total yields and are easily separable by column chromatography.5d The very long reaction times are evidence of the low DA reactivity of sulfinyl dienes 9, 10, even though the presence of the methoxy substituent appears to significantly increase the reaction rate.It is evident (Table 2, entries 16-18) that the endo/exo diastereoselectivity was low in these cases, and for diene 9d (Y = H) quite in favor of the usually less-favored exo isomers.A very good facial diastereoselectivity was observed for the end-o approach while both exoisomers were obtained in almost equal amounts.A tentative rationalization of these results resides in the high steric requirements of both the diene and dienophile, such that the less sterically congested exo approaches occur in high percentage, without significant facial discrimination, while the endo approach, which is much more sterically demanding, requires an almost complete facial selection.This last aspect is very useful from the synthetic point of view.We suggest that the formation of the only endo adduct 55 comes from the NPM approach to the Si face of the (S S )-diene 10e (Y = OMe) in its (m) conformation.Concordantly, the (R S )dienes 9d (Y = H, OMe) cyclo-add in endo fashion, mainly from their Re face in conformation (l).
Previously we had observed preferential endo approach to the opposite face of analogous sulfinyl dienes 5 and 6, but the conformational preferences proposed in these cases [analogous to (n) and (o)] can be prevented here by electrostatic repulsion between the sulfinyl oxygen and the πsystem of the fused benzene ring.With the hope of gaining experimental support for the arguments given above, we oxidized diene 10e (Y = OMe) to 4-{1-[(1S-exo)-2-bornylsulfonyl]vinyl}-1,2-dihydro-7-methoxynaphthalene following the usual procedure with m-CPBA, and submitted this sulfone, without isolation, to DA cycloaddition with NPM.The DA procedure was performed in CH 2 Cl 2 at room temperature for eight days, and then the reaction mixture, dissolved in 1,2-dichloroethane, was maintained at reflux for a further eight days, but unreacted diene and dienophile were recovered.The failure of this sulfone cycloaddition can be ascribed to its concurrent steric and electronic requirements which prevent fruitful DA approaches with NPM under the reaction conditions adopted.
As a part of our investigation on the behavior of enantiomerically pure sulfinyl -1,3-dienes in DA cycloadditions, we compared the reactivity of open-chain 1-and 2-p-tolyl sulfinyl dienes possessing a highly DA activating substituent such as the trimethylsilyloxy group, 17 and confirm our remark, made for the 1-and 2-sulfinylvinylcyclohexenes, 14 and 6ce, 6 regarding the influence exerted by the sulfinyl group's position on the conjugated system; 2-sulfinyl-dienes exhibit a higher reactivity in DA cycloadditions than 1-sulfinyl-dienes.
Hetero-Diels-Alder (HDA) reactions give easy access to heterocycles which constitute the structural skeleton of a large number of widespread natural compounds. 18The most common HDA cycloadditions have been directed towards the synthesis of pyran derivatives as carbohydrate precursors; in this, we have extended the use of our enantio-pure sulfinyl homoand hetero-dienes.
HDA reactions of 5bd with commercially available aromatic aldehydes were attempted under mild conditions and in the presence of Lewis acids.The results obtained, 10 were disappointing in many ways, and prompted us to study an electron-deficient aldehyde, ethyl glyoxalate, which represents a more reactive dienophile, and is more convenient for application of the corresponding pyranoid cycloadducts to the synthesis of enantiopure carbohydrates and carbohydrate-like products (Scheme 6). 8,19The facial diastereoselection was moderate (63% attack of the dienophile onto the preferred diene face; entry 5 in Table 3) even when performed in the presence of a suspension of LiClO 4 in CH 2 Cl 2 (entry 4).Further experiments (entries 3,6,7 in Table 3) were performed in the presence of different Lewis acids in CH 2 Cl 2 solution.The results obtained deserve some comments.When a very strong Lewis acid such as TiCl 4 was used, extensive decomposition of the reactants was observed, even at low temperature. 20nCl 2 or Eu(fod) 3 catalysis led to a reversal of the endo/exo diastereoselectivity in favor of the exo approach (compare entry 5 with entries 3 and 7 in Table 3).
This result can be attributed to catalyst sizes.Because Lewis acid co-ordination forces the dienophile into its s-cis conformation, the highly sterically demanding ZnCl 2 or Eu(fod) 3 can favor the less sterically congested exotransition state.A moderate enhancement of facial diastereoselection was observed in the Eu(fod) 3 catalyzed cycloaddition but the yield of the reaction was low.In Table 3 (entries 1, 2, 8, 9) the results of HDA reactions of dienes 5bd and 5cd with thiodienophiles are also reported.Although the most common HDA cycloadditions have been directed towards the synthesis of pyran derivatives as carbohydrate precursors, conveniently substituted thiopyrans (obtained by using the HDA approach) have received special attention as intermediates in the synthesis of biologically active agents. 21he lack of facial diastereoselectivity in the HDA cycloaddition of diene 5cd to thiobenzophenones (Table 3, entries 8 and 9) can be ascribed to the combined steric characteristics of the cyclohexene moiety of the diene and aryl substituents of the dienophile in the transition states of the cycloadditions. 22These high steric requirements, which are due in the dienophile to the nearly perpendicular disposition of the aryl groups with respect to the C=S plane, make unimportant the topological differentiation between the diene faces induced by the alkylsulfinyl auxiliary, and poor stereochemical results are observed.

Conversion of Diels-Alder cycloadducts into enantiopure cyclohexene derivatives
The high control exerted by the sulfoxide chirality on the stereochemistry of the final adducts such as 15 (Scheme 2), and the easy isolation of the cycloaddition products in high yields, prompted us to explore their usefulness in stereoselective syntheses of target molecules.With the aim of obtaining enantiopure polyhydroxylated molecules from (3R,4S,R S )-4-hydroxymethyl-1-[(1S)-isoborneol-10-sulfinyl]-3-methoxycyclohexene (82) (Scheme 8) we decided to use iodotrimethylsilane (TMSI) to cleave the O-Me bond, following a well-assessed literature procedure. 23The reaction afforded (R)-4-(hydroxymethyl)cyclohex-2-ene-1-one (83) 24 and the more mobile bis-{(1S,2R,4R)-2-hydroxy-7,7-dimethylbicyclo[2.2.1]hept-1-ylmethyl}disulfide (R*SSR*) 25 as the major products. 26In contrast to the disappointing result of losing the unmasked hydroxy substituent from the cyclohexene skeleton, the presence of a carbonyl function-which had taken the place of the alkylsulfinyl moiety-appeared to be an unexpected but remarkable outcome of the TMSI-induced reaction.It must be noted that alkyl(vinyl)sulfinyl cycloadducts frequently suffer the problem of ineffective removal of the chiral auxiliary by classical procedures, such as Raney nickel desulfurization.5c,27

Scheme 8
We have explored the reactivity of TMSI with 1-sulfinylcyclohexene adducts other than 15 and/or its derivative 82.For instance, compounds 42 and 50 led, respectively, to (84 + 85) and 86 in the presence of the halogenated reagent (Scheme 8), while the reaction of TMSI with cycloadduct 53 afforded (3aS,11aS)-7-methoxy-2,3,3a,4,5,10,11,11a-octahydro-2phenylnaphthalene-[2,1-e]-isoindole-1,3-(1H)-dione (87), which may be the thermodynamically favored product of the reduction and dehydration of an intermediate carbonyl compound transiently formed by TMSI-promoted conversion of the α,β-unsaturated sulfoxide 53.5d Moreover we have studied the behavior of simple open-chain vinyl sulfoxides in the presence of TMSI, owing to our interest in testing the generality and efficiency of the procedure, and consistent results were obtained. 26he conversion of vinyl sulfoxides into aldehydes or ketones has the advantage of transforming the C-S cleavage of vinyl sulfoxides (which is generally not easy to achieve), in the functionalization of the molecule with a carbonyl group which can be subjected to numerous synthetic transformations.To the best of our knowledge, this conversion is without precedent in the literature.All of these considerations allow us to anticipate many applications which will be developed around this reaction, and further studies in this direction are now in progress.The cycloadduct 19 (R* = e) was also converted into the polyfunctionalized enantio-pure cyclohexanone 88 by a sequence of highly stereoselective reactions (Scheme 9).The cyclic vinyl sulfone 89 was obtained in high yield from cycloadduct 19 by m-CPBA oxidation.The reaction of sulfone 89 with t-BuOOLi uniquely afforded the epoxide 90, showing complete diastereoselectivity. Epoxide 90 was used without further purification in the next and final step of the process.Epoxide ring opening and concomitant loss of the alkylsulfonyl group were achieved by using MgBr 2 .Et 2 O, and the diastereoselection was complete.5c

Conclusions
The synthetic strategy based on the addition of sulfenic acids to triple bonds conjugated with double-homo-and hetero-bonds has been applied successfully to the preparation of enantiopure sulfinyl homo-and hetero-dienes, which are not easily accessible by other synthetic routes.A large number of monoterpene systems, belonging to the chiral pool, can be used as starting products in this methodology.In particular, the good yield in DA cycloadducts, easy separation of diastereomeric mixtures, and crystallinity of the final products demonstrate the utility of the camphor skeleton in the design of chiral auxiliaries based on sulfoxides.

Table 1 .
Cycloadditions of sulfinyl dienes 5 and 6 with acyclic homo dienophiles a a Identification by numbers of cycloadducts in Scheme 2 and Table1is independent from R* features.bFigure referring to both exoadducts as a whole.cendo-/exo-Selectivity refers to substituents at C-3 and C-4.Issue in Honor

Table 2 .
Cycloadditions of sulfinyl dienes 5, 6, 9, 10 with cyclic homo dienophiles a a Identification by numbers of cycloadducts in Scheme 4 and Table2is independent from R* features.

Table 3 .
Cycloadditions of sulfinyl dienes 5 with hetero-dienophiles a The identification by numbers of the cycloadducts in Scheme 6 and Table3is independent of the R* features.b Extensive decomposition of reactants was observed.