STEREOCHEMICAL CONTROL IN INTRAMOLECULAR DIELS-ALDER REACTIONS WITH CARBENE COMPLEXES AS ESTER SYNTHONS

An evaluation of the intramolecular Diels-Alder reactions of Fischer c arbene complexes was made for complexes which have an all-carbon tethe r between the diene and dienophile. Specifically, both the cis- and tr ans-isomers of the deca-2,7,9-trienyl carbene complexes (26-cis, 24-tr ans) and the undeca-2,8,10-trienyl carbene complexes (27-cis, 25-trans ) were prepared and the rates and stereoselectivities of their intramo lecular Diels-Alder reactions were in each case compared with those of the known reactions of their corresponding methyl esters. For the dec a-2,7,9-trienyl complexes 24 and 26, the stereoselectivities are compa rable to those observed for the Lewis-acid-catalyzed reactions of thei r corresponding methyl esters and much higher than the thermal reactio ns of the methyl esters which are completely unselective. The undeca-2 ,8,10-trienyl complex 25 undergoes intramolecular Diels-Alder reaction with a 93:7 endo/exo selectivity whereas the corresponding methyl est er 7 is known to give a 51:49 selectivity under thermal conditions and to fail with attempts at Lewis acid catalysis. The cis-substituted co mplex 27 also undergoes a selective reaction where the corresponding r eaction of the ester fails. In addition no trace of isomerization of t he cis-complex 27 could be observed during its cycloaddition. The trip henylphosphine and isopropoxy complexes 28-30 were prepared, and the s tereoselectivity of their intramolecular Diels-Alder reactions is cons istent with an s-trans conformation of the vinyl carbene funtionality in the transition state. In all cases the carbene complex cycloadducts can be oxidatively cleaved in high yield to their corresponding ester s. The results show that the value of these complexes as synthons for esters is a result of tolerance of the carbene complex functional grou p to sensitive diene units which do not tolerate traditional Lewis aci ds utilized in accelerating intramolecular Diels-Alder reactions.