Dedicated to Professor Manfred Braun on the occasion of his 70th birthday
Abstract
Two routes to 3-alkyl-substituted 1-alkoxyallenes are investigated. The deprotonation
and alkylation at C-3 of methoxyallene requires prior silylation at C-1 and provides
mixtures of the expected products and 1-alkyl-substituted methyl propargyl ethers
as minor component. The desilylation of these mixtures affords the desired 3-alkyl-substituted
1-methoxyallenes together with the 1-alkyl-substituted isomers in moderate overall
yields. Following the second route, the disadvantages of this three-step method are
avoided. In analogy to Brandsma, C-3 alkylation of methyl propargyl ether and subsequent
isomerization under basic conditions affords two of the desired 3-alkyl-substituted
methoxyallenes in good yields. This method is also applied to propargyl ethers bearing
a diacetone-fructose-derived auxiliary. Two diastereomeric 3-alkyl-substituted 1-alkoxyallenes
are formed during the isomerization, the ratio being strongly dependent on the reaction
conditions. The mechanistic aspects of these observations are discussed on the basis
of deuteration experiments and the configurational stability of the ambident propargyl-allenyl
carbanion involved.
Key words
alkoxyallenes - alkynes - lithiation - alkylation - carbohydrate derivatives
