Pyridinium chlorochromate chemistry. New insight into oxidation of tetrahydrofurans

A thorough investigation of the minor oxidation products of two penta-tetrahydrofuran compounds with pyridinium chlorochromate has been carried out. Isolation of ring-B oxygenated spiroketal and degradation products, including polycyclic mono-and bis-lactone compounds, supports the previously postulated involvement of cyclic enolether intermediates in the oxidation of THF and poly-THF substances with PCC. Based on the collected evidence, a new mechanistic route for the PCC-mediated oxidative cleavage of  - hydroxy-THF compounds to  -lactones has been postulated. The proposed mechanism well agrees with the one reported for the oxidative cleavage of 8-hydroxy-neoisocedranol oxide by RuO 4 , a fact that further supports our previous observation on the similar oxidizing behaviour shown by PPC and RuO 4 towards THF-containing compounds.


Introduction
Pyridinium chlorochromate (PCC) is a well known oxidizing reagent employed in a number of processes. 1,2hough the most popular process mediated by PCC is the oxidation of primary and secondary alcohols to aldehyde and ketones, respectively, many other functional groups undergo PCC oxidation.As a continuation of our interest in oxidative processes mediated by transition metal oxo-species [3][4][5][6][7][8][9][10][11] as well as in the synthesis and derivatization of new THF-containing compounds, [12][13][14][15][16][17][18][19][20][21] we recently focused on oxidation processes mediated by PCC.These studies led to the discovery of new interesting transformations including oxidative spiroketalization, 22 oxidative cleavage of trisubstituted mono-THF compounds 23 and synthesis of bis-acyloxy-1,4-and 1,5-diketones from THF and THP-diols, respectively. 24During these studies evidence on some similarities about the oxidative behaviour of PCC and RuO4 were also recorded.In particular, we have previously reported 22 that penta-THF 1 17,20 (Scheme 1), when treated with PCC furnishes the new compounds 2-8 four of which (5-8) representative of a novel class of poly-THF spiroketals displaying antitumor activity on breast and ovarian cancer cell lines. 22heme 1.A new type of spiroketal compounds obtained by PCC-mediated oxidation of penta-THF compound 1.
Inspection of the structure of these substances reveals that compound 1 undergoes three types of oxidative processes all involving the interaction of the oxidant with different THF ether methine groups: a) The oxidative cleavage of the C2-C3 and/or C22-C23 bonds in 1 give rise to lactones 2 and 4, the main oxidation products; b) The oxidative spiroketalization, formally involving the C(2)OH group and the C-7 carbon at the cis-cis bis-THF terminus of the poly-THF chain, gives rise to the spiroketal-containing compound 5, possessing the intact carbon skeleton of 1.The terminal lactone in the related spiro-compound 6, in turn, arises from the further oxidative cleavage of the C22-C23 bond in 5, according to the route highlighted in point a; c) The inter-THF C-C oxidative bond cleavage in 5 and/or 6 is responsible of the formation of the degraded, minor, spiro-compounds 7 and 8.The transformations highlighted in points a and b point out the similarity of the oxidizing behaviour of PCC with the one displayed by RuO4 which catalyzes similar processes by reaction with 1. 13 The PCC-mediated conversion of -hydroxy mono-and poly-THF compounds to -lactones (see for example conversion of 1 to 2-4 or 5 to 6 in Scheme 1) is a well-documented process 14,17,[25][26][27][28][29][30][31] although no definitive proof on the cleavage mechanism has been provided.On the other hand, spirocyclization leading to compounds 5-8 is a novel transformation which is attractive both from a mechanistic and applicative point of view.Therefore, we were interested to collect further evidence on these processes and, more in general, on the other oxidative routes depicted in Scheme 1.To this end, a careful analysis of the minor side-products formed during the oxidation of 1, and the corresponding dibenzoate, with PCC was undertaken.This survey allowed the identification of novel pathways working in the oxidation of differently substituted tetrahydrofurans with PCC, or allowed to confirm previously formulated mechanistic hypotheses on some PCC-mediated processes. 22,23In addition, based on the acquired evidence, we could formulate a new plausible mechanistic route on the PCC-mediated oxidative cleavage of THF rings bearing a  tertiary alcohol function, 14,17,[25][26][27][28][29][30][31][32][33] such as 1 and 2, to the corresponding -lactones, that well agrees with the mechanism proposed for the analogous process mediated by RuO 4.

Results and Discussion
C-8 oxygenated spiroketals.Penta-THF 1 was synthesized from squalene according to a previously described procedure 17 and its purity checked by direct and reversed-phase HPLC and high-field NMR analysis to exclude the presence of minor impurities.The oxidation of 1 in the presence of PCC was carried out in CH2Cl2 at reflux as previously described. 22The crude reaction mixture was subjected to a careful preparative and, when required, to analytical HPLC to give the new C-8 oxygenated spiroketals 9 and 10 (Scheme 2) as well as some poly-THF dilactone compounds (12-14, see later Scheme 4) derived from the oxidative degradation of the carbon skeleton of 1, besides previously isolated compounds 2-8 (Scheme 1).Scheme 2. Minor C-8 oxygenated spiroketal compounds obtained by PCC-mediated oxidation of 1.
Determination of the stereostructure of the 8-oxo-compound 9 was accomplished by registration of a full set of high-field 2D-NMR spectra while the structure of the corresponding C-8 alcohol 10 was inferred by chemical correlation with 9 and NMR evidence.In particular, the configuration of the C-7 spiro-centre belonging to the tricyclic spiroketal subunit in 9 was established to be the same found in related spirocompounds 5-8 (Scheme 1) through unambiguous ROESY evidence (Figure 1).
The structural relationship between 9 and the corresponding alcohol 10 was proven by sodium borohydride reduction of 9 that gave alcohol 10 (10%, Scheme 3), through attack of the reducing agent to the carbonyl plane from its upper face, along with the corresponding triol 11 (75%), derived from the further reduction of the -lactone function in 9. Configuration at C-8 in 10 was established based on 2D-NMR studies carried out on the corresponding triol 11, available in higher amounts, having ascertained the sterostructural relationship between the two compounds by borohydride reduction of the former to the latter (Scheme 3).In particular, a significant ROESY correlation peak between Me-26 and H-8 (Figure 1) established the -orientation of the OH group in this compound and thus its cis stereostructural relationship with the oxygen bridging C-2 and C-7 in the F ring, a fact having important mechanistic implications, as discussed later.Dilactones.Dilactones 12-14 (Scheme 4), derived from the cleavage of the poly-THF skeleton of 1 at different hydrogen-carrying ether positions, were also isolated as minor products (overall 2% yield) from the oxidation of 1 with PCC.Their stereostructures were determined by 2D-NMR studies, including ROESY experiments.The trans configuration of both the THF rings in the most abundant compound 12 (1%), indicated it to originate from the oxidative cleavage of 1 at the C6-C7 bond connecting A and B THF rings accompanied by the oxidative removal of the terminal 2-hydroxypropyl moiety at the other end of the molecule (C22-C23 bond cleavage).Similarly, dilactone 13 (0.5 %), the threo-cis-threo-trans-threo isomer of 12, originated from the analogous oxidative cleavage of both the C18-C19 bond, connecting D and E THF rings, and the oxidative cleavage of the C2-C3 bond.Conflicting NMR evidence prevented assignment of the configuration of the remaining tricyclic dilactone 14 (0.5%).Therefore, both threo-cis-threo and threo-trans-threo configurations are possible for this substance, according to two cleavage modes namely cleavage of C2-C3/C14-C15 or C10-C11/C22-C23 bond pairs, respectively.

Scheme 4. Minor polycyclic dilactones from the PCC-mediated degradation of 1 and relevant cleavage patterns (see arrows).
A mechanistic rationalization for the formation of spiroketals 9, 10 and dilactones 12-14.Isolation of the above products suggests new interesting oxidative pathways.A plausible mechanistic rationalisation for the formation of above spiroketal and dilactone substances that agrees with reported reactivity of PCC 1,2 and previously developed chemistry by our own group, 22 is given in schemes 5, and 7-9.
Initially, interaction of PCC with the C(2)-OH group gives rise to chromium ester 15 (Scheme 5).Thus, the oxo-chromium appendage tethered to C-2, in this intermediate, may attack the close-in-space C7-H bond, causing the closure of the spiroketal function of 5 (route a), with expulsion of a chromium species.Next, the oxidative cleavage of the C22-C23 bond in the latter generates the related terminal lactone 6.However, an alternative route in which a cyclic chromium diester intermediate 16 is formed from 15 through the [3+2] addition of its O=Cr=O portion to the C7-H bond (route b), appears plausible.Scheme 5. A mechanistic route explaining the formation of spiroketals 5 and 6.
The preliminary formation of chromate ester 15 is a reasonable assumption since there is evidence of the very rapid formation of such esters by reaction of PCC with primary, secondary and tertiary alcohols. 34n the other hand, formation of chromium diester 16 from 15 is supported by a number of previously reported evidence.Attack of transition metal oxo-species such as RuO4 [35][36][37][38] and OsO4 [39][40][41] to C-H bonds of alkanes is well-known and has been suggested to proceed through [3+2] addition of a C-H bond across an O=M=O unit through a mechanism analogous to the one now widely accepted for alkene bis-hydroxylation.In addition, it has been reported that the RuO4 oxidation of neoisocedrane oxide 42 (Scheme 6), a THF-containing sesquiterpene, proceeds much in the same way we hypothesize in the transformation of 15 to 16, through the insertion of an oxoruthenium bond into the ether C-H bond of the THF ring, successive to the formation of a ruthenate ester, aspecies analogous to 15. Conversion of 15 to 16 seems also plausible based on the similar oxidizing behaviour exhibited by RuO4 and PCC towards 1, recently disclosed in a study carried out in our group. 13,22ormation of diester 16, well explains the formation of compounds 9, 10 and 12, where the oxidation of the B-ring is required.In fact, 16 can be envisaged to evolve through two routes.In particular (Scheme 7), oxidative cleavage of the C6-C7 bond connecting A and B rings, would be responsible of the formation of intermediate mono-lactone 17 from which dilactone 12 would then be formed by cleavage of the C22-C23 bond.4][45] Although this compound could not be detected among the oxidation products of 1, the benzoate of 18 (see later compound 32, figure 3) has been isolated as a minor product when the dibenzoate of 1 was oxidised in the same conditions. 23A chromium diester intermediate strictly similar to 16 has been postulated in the oxidative ring fission of 2,5-dialkylfuranes with PCC to give -unsaturated 1,4dicarbonyl compounds. 46heme 6. Oxidative cleavage of 8-hydroxy-neoisocedranol oxide with RuO4 based on literature data [42].
Alternatively, an elimination step in 16 can generate the ring-B enolether intermediate 20 (Scheme 8), where a chromium-containing appendage is still linked to C-2 and the carbon skeleton of 1 is preserved.8][49] It is worth noting that such a cycloaddition step also explains the observed cis relationship between the ring-F spiroketal oxygen and the C-8 hydroxyl group (Figure 1).
Our hypothesis on the spiroketalization step involved into the conversion of 20 to 10 well agrees with the McDonald et al. 48and Schlecht et al. 49 mechanistic proposals.This transformation is strongly reminiscent of the oxidative spirocyclization of cyclic enolethers mediated by rhenium (VII) oxides reported by Boyce and Kennedy 50 and would represent the first example of the PCC-induced formation of a cyclic spiroketal involving an enolether double bond.It is to be noted that this transformation involves the chromium ester of a tertiary alcohol (C2-OH) while the rhenium-mediated process is reported to induce the sole spiroketalization of primary alcohols.This transformation is certainly worth of further studies using ad hoc devised substrates.
Formation of dilactone 13, the cis-trans isomer of 12, would proceed in a way strictly analogous to that leading to 12, by cleavage of C18-C19 and C2-C3 bonds, via intermediates 22 and 23 (Scheme9).However, in this case, due to the threo-trans-threo configuration of the bis-THF terminus (Figure 2), an intermolecular attack of PCC to C18-H bond is to be invoked because a C23-tethered oxochromium appendage and the C18-H bond cannot be brought near in the space, as it happens in 15 (Scheme 5), This spatial arrangement also explains why no spiroketalization is observed at this terminus but only cleavage of the C22-C23 bond to give 2. Scheme 9. Proposed route for the formation of dilactone 13.Finally, formation of dilactone 14 from 1 is depicted in Scheme 10.It requires the cleavage of one of the bonds adjacent to the central THF ring (C10-C11 or C14-C15) to take place (the attack at the C11-H bond is shown in scheme 9).An inter-molecular attack of PCC is required in this case as well, to give the chromium ester intermediate 24 which then gives rise to bis-lactone 14 via mono-lactone 25.A bis-THF enolethercontaining species 26 would once again be produced in this step, the fate of which we were unable to follow further.
Summarising, the above results clearly established the ability of PCC to cleave inter-THF bonds in 1. 23 The main oxidative routes, leading to -lactones 2-4, 17,22 spiro-compounds 5 and 6 as well as minor C-8 oxygenated spiro-compounds 9 and 10, and bis-lactone 12 (Schemes 1, 5, 7 and 8) proceed through the preliminary formation of chromium esters by interaction of PCC with the two hydroxyl groups of 1. Secondary routes leading to truncated spirolactones 7 and 8 (Scheme 1), dilactones 13 (Scheme 9) and 14 (Scheme 10), likely proceed through the inter-molecular attack of PCC to suitable THF CH bonds followed by inter-THF C-C cleavage.There is evidence indicating the formation of cyclic enolethers.This also further supports our recently formulated hypothesis on the involvement of an enolether species in the oxidative cleavage of trisubstituted THF rings with chlorochromatoperioadate (CCP), an oxidizing agent generated by reaction of PCC and periodic acid. 23heme 10.Proposed mechanism for the formation of dilactone 14.
Minor oxidation products from the PCC-mediated oxidation of penta-THF dibenzoate 27.Previously, we have investigated the reactivity of compound 27, the bis-benzoate of 1 (Scheme 11), in the same oxidative conditions used for the oxidation of 1. 23 Because of the absence of free C-2 and C-23 hydroxyl groups in 1, the main oxidative routes giving terminal lactones 2-4, as well as spirocompounds 5-8 were depressed and the intermolecular attack of PCC to the internal THF rings in 1 was enhanced.This resulted in the formation of compounds 28 and 29 as the major products.We hypothesized that they could originate by the oxidative cleavage of ring-B or D enolethers. 23In addition, ketol 30, embodying a ring-B oxygenated moiety, lactone 31, lacking one of the terminal rings of 27, and fragment 32, were also isolated as minor products.Formation of these compounds could be rationalised through involvement of ring-B or D enolethers, as well.Isolation of compounds 28-32 suggested the preferential attack of PCC to B and D rings in 27.A reinvestigation of the oxidation of 27 with PCC,, in the previously tested conditions, has now led to the identification of further four minor products originating from the attack of the oxidant to the central C THF ring.Their formation has been explained through mechanistic paths in line with those formulated for all the other reaction products.In particular, the two isomeric, small-sized, lactones 33 and 34 (major isomer 4%; minor isomer 2%) and the two isomeric unsaturated aldehydes 35 and 36 (1% each), were isolated.A new proposed mechanism for the oxidative cleavage of -hydroxy-THF compounds.Conversion of 1 to 2-4.The PCC-mediated oxidative cleavage of -hydroxy mono-and poly-THF compounds to -lactones (see for example conversion of 1 to 2-4 or 5 to 6 in Scheme 1) is a well-documented process [25][26][27][28][29] interesting both from a mechanistic and applicative point of view.6][27][28][29] In the light of the above collected evidence a comment to this transformation, with reference to THF bearing-tertiary alcoholic moieties, seems appropriate.When reasoning for example for the transformation of 1 to 3 (Scheme 1), a summary of the reported routes, explaining the oxidative cleavage of the C2-C3 bond, is shown in Scheme 13.
In particular, the great part of the reported hypotheses supposes a preliminary coordination of PCC to the alcohol group to the THF ring.In accord with the route proposed by Stark et al. 25 for the strictly related oxidative cleavage of THF and THP alcohols to -and -lactones, the C-2 tethered oxochromium appendage in the first-formed species 15 may attack the C-3 carbon though route a, with formation of a C-3 chromium ester intermediate 39 that, as such or in a different oxidation state, would generate the lactone function in 3.Alternatively, a cation species 40 17,25 could be formed (route b) that is then further oxidised by PCC to give 3. Another reported route (route c) 28 supposes the involvement of an enolether species such as 41, formed by dehydration, that is then cleaved by PCC.However, such a type of substance could not be detected among the reaction products, as the authors pointed out. 28In line with our reasoning on the conversion of 15 to 16 shown in Scheme 5, we believe that a fourth plausible path (Scheme 14) can be proposed where the C3-H bond is attacked by the close-in-space C-2 tethered oxidant, to give the cyclic chromium diester 30.Successive oxidative fragmentation of this intermediate, would proceed in the usual manner with expulsion of acetone, generating the lactone function of 3. Note that this route would compete with the one where the C7-H is attacked by the same C-2 oxochromium portion to give 16, as shown in Scheme 5. Cyclic esters such as 42 are thought to be involved into the oxidative cleavage of alkenes or vicinal diols with related oxo-species RuO4, OsO4, RuO4 -and MnO4 -.Importantly, our path is strictly similar to the proposed route for the oxidative cleavage observed for 8-hydroxy-neoisocedranol oxide with RuO4 shown above in Scheme 6, 42,3,13, which we also postulated to work in the ruthenium tetroxide catalyzed oxidative cleavage of strictly related poly-THF spiroketal compounds. . 13[27][28][29] Scheme 13.Oxidative cleavage of the -hydroxy-THF portion of 1 according to previously formulated mechanistic hypotheses.Scheme 14.Our proposed route for the oxidative cleavage of the -hydroxy-THF portion in 1.

Conclusions
The data collected in the present study expand our knowledge on the oxidative behaviour of PCC with poly-THF compounds under classical conditions and may help to understand the reactivity of PCC with simpler ether compounds.Penta-THF 1 revealed itself as a good model compound to study the action of PCC on adjacently linked poly-THF compounds.Novel oxidative pathways leading to degradation, or oxidative modification, of the poly-THF skeleton, with formation of new types of poly-THF compounds, have been disclosed.Plausible hypotheses, consistent with the known reactivity of PCC, have been presented to explain the formation of all isolated substances.In particular, attack of the oxidant at the various THF C-H bonds, either in an intra-or inter-molecular way, is thought to be the first event, which leads to formation of a chromium ester intermediate.The main products originate from the first-formed chromium ester species with the alcohol functions in 1. Inter-THF bond cleavage, to give small-sized dilactones, takes place in a little extent by intra-molecular attack of PCC to THF rings.Isolation of minor C-8 oxygenated spiroketal compounds strongly suggests that a cyclic ring-B enolether is involved in their formation.We believe that further experimental work on the chromium-mediated spiroketalization of cyclic enolethers, including -tethered tertiary alcohol portions in their structure, is certainly desirable.Oxidation of dibenzoate 27 led to degraded compounds the formation of which is explained through the intervention of ring-C enolethers in this case as well.Additional support to the previously observed similarity of the chemical oxidative behaviour of PCC and RuO4 towards THF-containing substances, 13 has been provided.Further clarification of this issue could come from theoretical studies.

Experimental Section
General.All reagents were purchased at the highest commercial quality and used without further purification.Reactions were monitored by thin-layer chromatography carried out on precoated silica gel plates (Merck 60, F254, 0.25 mm thick).Merck silica gel (Kieselgel particle size 0.063-0.200mm) was used for column chromatography.Na2SO4 was used as a drying agent for aqueous work-up.HPLC separations were carried out on a Varian 2510 apparatus equipped with a Waters R403 dual cell differential refractometer using Phenomenex 250 x 10 mm and 250 x 4.6 mm (both 5) and LiChrosorb RP-18 250 x 4.0 mm columns.NMR experiments were performed on Varian Unity Inova 700, Varian Unity-Inova 500, Varian Mercury Plus 400, Gemini 200 spectrometers in CDCl3.Proton chemical shifts were referenced to the residual CHCl3 signal (7.26 ppm). 13C-NMR chemical shifts were referenced to the solvent (77.0 ppm).J values are given in Hz.Abbreviations for signal coupling are as follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet.IR spectra were recorded on a Jasco FT-IR 430 spectrophotometer.The High Resolution MS were recorded on a Bruker APEX II FT-ICR mass spectrometer using electron spray ionization (ESI) technique.For all the reported products the numbering previously given 17 for the penta-THF 1 is used.

double bond of 20 .Scheme 8 .
Scheme 7. A mechanistic explanation for the formation of bis-lactone 12.

Figure 2 .
Figure 2. Fate of the postulated chromium ester formed at the terminal trans-threo-trans bis-THF portion of 1.