Mechanism of Lithium Perchlorate/Diethyl Ether-Catalyzed Rearrangement of α- and β-endo- and -exo-Dicyclopentadienyl Vinyl Ethers:  Use of Deuterium Labeling and a Chiral Probe

Abstract

Lithium perchlorate/diethyl ether (LPDE) mediated rearrangement of α- and β-endo-dicyclopentadienyl vinyl ethers 5 and 6 resulted in the formation of the aldehyde 8, indicating that the mechanism is nonconcerted and the recombination of the ion pair occurs at the dissociated stage and not the intimate ion-pair stage. Proof of this came from deuterium-labeling studies and the use of an optically pure starting material. Furthermore, that the ionic intermediate formed must be symmetrical resulting in enantiomeric aldehydes from both normal and allylic attack corresponding to products of formal 1,3 and 3,3 shifts was seen in the chiral analysis of the benzoate derivative of the aldehydes formed from the optically pure vinyl ether.

Thermal transformation of 1σ and 1β-endo and exo- dicyclopentadienyl vinyl ethers (competitive Claisen and Cope systems)

“…Our study has been shown that 1β-vinyl ether 1b undergoes Cope rearrangement in preference to Claisen rearrangement, furnishing the anti ether 2c 5 .This preference for Cope rearrangement has also been observed with a few other γ-substituted vinylethers .In none of these cases, any Claisen rearrangement was observed. In the dihydro series where there is no possibility of Cope rearrangement, the 1α-vinylether 1c underwent Claisen rearrangement, where as the 1β-vinylether 1d remained unchanged even after refluxing for a long time .…”

“…Thermolysis of 1α− vinyl ether 1a 5 in refluxing xylene for 3 h furnished a mixture of two aldehydic products 2a and 2b in 2:1 ratio in 70% yield, which are closely resolved in tlc. When 1a was refluxed in toluene for 6 h, it gave exclusively only the aldehyde 2a 5 in 70% yield (based on recovery of starting material).…”

“…When 1a was refluxed in toluene for 6 h, it gave exclusively only the aldehyde 2a 5 in 70% yield (based on recovery of starting material). Attempts to force this reaction to completion by prolonged heating led to the formation of 2b viz., product of sequential Claisen -Cope rearrangement demonstrating that Claisen rearrangement is preferred over Cope rearrangement in this system, which parallels literature reports 3 .…”

“…As a result of this, attainment of a six membered cyclic transition state is geometrically impossible in this system and hence there is no possibility of Cope rearrangement 11 . We observed that the 1α-exo-dicyclopentadienyl vinyl ether 1g underwent a clean Claisen rearrangement and furnished the aldehydic product 2f in 75% yield 5 . Under identical conditions the epimeric 1β-vinyl ether 1h remained inert and only the starting material was recovered while under drastic conditions i.e., refluxing in high boiling solvents like p-cymene, o-dichlorobenzene…”

Chemoenzymatic Synthesis of an Enantiomerically Enriched Bicyclic Carbocycle Using <i>Candida parapsilosis</i> ATCC 7330 Mediated Enantioselective Hydrolysis

“…Various chemical [1] [22] and biocatalytic [26]- [29] methods are available for the synthesis of enantiomerically pure aryl/alkyl-allyl alcohols. Lipases, generally obtained from microorganisms, are the most widely used enzymes in organic synthesis, mainly due to their high enantioselectivity [30]- [32], the possibility of using nonconventional solvents like organic solvents and ionic liquids [33] and are economical, stable, and easy to reprocess. Among the methods reported, only few specific examples are available for the lipase mediated resolution of aliphatic cyclic allylic alcohols.…”