Allenyl carbinol esters

Gagosz reported a mild and efficient protocol for the synthesis of 1,3-butadien-2-ol esters from allenyl carbinol esters. The methodology proceeded smoothly using 1 mol% of gold complex XXIV, at room temperature and some of the compounds were fully converted after only five minutes. The. E-isomer was preferably obtained in most cases (Scheme 16.45). 

Scheme 16.45 Isomerisation of allenyl carbinol esters to l,3-butadien-2-ol esters.

Recently, allenyl carbinol esters have also been efficiently rearranged into buta-1,3-dien-2-ol esters via metal catalyzed transformations. Gold (1) catalysis has proven efficient under smooth conditions (CH2CI2 at room temperature) when using the biphenylphosphme-based catalyst 173. The stereoselectivity of the process depends on the substrate and favors the formation of the E isomer (Scheme 73) [157]. More recently, the use of rhodium catalysts in similar transformations has been reported to lead to higher E selectivities, although in harsher reaction conditions (Toluene at 120 °C) (Scheme 73) [158]. 

Scheme 73 Metal catalyzed isomerization of allenyl carbinol esters into buta-l,3-dien-2-ol esters...

Allenyl carbinol esters 60 (Scheme 35) form l,3-butadien-2-ol esters 61 under mild conditions with low catalyst loadings and good -selectivity [195]... 

Scheme 35 Gold(l)-catalyzed reaction of allenyl carbinol esters...

Esters of allenyl carbinols give 2-acyloxy-l,3-dienes on treatment with AuNTf2, which is complexed to (2, 4, 6 -triisopropyl-2-biphenyl)dicyclohexylphosphine. ... 

Two synthetically interesting processes are the Sn2 reaction of the esters of allenyl carbinols with organozinc reagents to generate conjugated dienes, and the benzologue version (1,5-transpositional displacement) involving aUyltributylstannane. ... 

Rearrangement of allenyl carbinols by the ortho ester variation with trimethyl orthoacetate yields ( , )-dienes 20 together with small amounts of (Z.Z)-dienes 21 with ds > 90 % via chair transition state A, as long as R1 is a methyl group or hydrogen. When silicon-substituted substrates 18c and d are rearranged, the amount of 21 increases. This increase in 21 can be explained by transition state B. However, boatlike transition state C seems to be more likely because of favorable attraction between the methoxy and silyl groups662. 

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