Reactions and reactivity of allyl and related carbamates.
Carbamate chemistry has flourished since the 1980's which saw the introduction of the carbamoyl group by Hoppe and co-workers, prominent researchers in this area, as a suitable moiety for the activation and stabilisation of alpha-heterocarbanions which are important intermediates in synthetic organic chemistry. This functionality increases a-CH acidity and stabilises the a-lithiocarbanion thus generated. The versatility of the carbamate group has spawned investigations into the numerous possibilities that exist for the functionalisation of the carbon alpha to the carbamate group. Some of these possibilities were explored in the present work. The direct formation of stable benzylic carbanions alpha to an oxygen atom is not a facile process as such lithiated species are unstable and readily undergo a 1,2-Wittig rearrangement. However, protection of the alcohol as the carbamate enabled functionalisation of both benzylic sites with a variety of electrophiles. Benzyl carbamates were found to react regioselectively with a,~-unsaturated electrophiles to afford either the 1,2- or l,4-addition products. The regioselectivity could be controlled by the presence or absence of a silyl moiety, either a trimethylsilyl or tertbutyldimethylsilyl moiety, attached at the benzylic position, alpha to the carbamate. The reaction of benzyl carbamates with 2-cyclohexen-1-one and 2-butenolide afforded the 1,2-addition adducts in moderate to good yields (25-66%). The isolation of the masked ahydroxy ketone resulting from the opening of the lactone ring on reaction of 2-butenolide with benzyl N,N-diisopropylcarbamate was surprising, as this electrophile is known to behave as an excellent Michael acceptor. The introduction of a silyl moiety at the benzylic position facilitated conjugate addition, yielding the corresponding Michael addition products. A novel cyclisation was observed on reaction of a-silyl benzyl carbamates with methyl acrylate to give the a,y;y-trisubstituted butanolides (y-butyrolactones). The proposed vii mechanism of this reaction is discussed and illustrates the migrational ability of the carbamate functionality. Chiral induction in benzylic systems has only been investigated by a few authors. In the present investigation two possible routes to chiral induction at the benzylic position were investigated using either a (-)-sparteine-mediated transformation or chiral carbamates derived from (L)-proline. (-)-Sparteine mediated transformations were solvent dependent with the best enantiomeric excess of 28% being obtained. The results indicated that racemisation at the benzylic position was faster than reaction with an electrophile. The chiral carbamates were only slightly more successful yielding the a-substituted adducts with diastereoselectiVities of the order of 20-50%. The diastereoselectivities were found to be enhanced by the addition of HMPA which coordinated with the transition state blocking one of the faces to attack by the electrophile. The 1A-addition reaction with 2-butenolide was particulary useful as it allowed the exploration of the synthesis of lignan derivatives from benzyl carbamates. Lignans are secondary plant metabolites and exhibit.a wide range of biological activities. Applying the methodology developed to the synthesis of lignans resulted in the preparation of an arylnaphthalene lignan, in 79%, and the partial synthesis of racemic fargesin, a racemic furofuran lignan. This natural product synthesis highlighted the difficulty in removing the carbamate functionality and re-introducing the alcohol moiety. Thus, hydrolytic cleavage of the benzyl carbamates was investigated using a selection of mineral acids. Depending on the reaction conditions, the products were found to be the corresponding benzyl alkanoates or benzylhalides. The alkanoates are derived from nucleophilic displacement of the initially formed benzyl halide A novel deprotection of a-silyl benzyl carbamates was also observed in which the carbamoyl group was cleaved to afford the a-silyl alkanoates and chlorides without the loss of the silicon moiety.