Biological reaction, alcohol characteristics

A more extensive investigation of the matching and mismatching characteristics of allenylzinc reactions was conducted by Marshall and Schaaf19. The thrust of their studies focused on the effect of a /1-OTBS stereocenter on the stereoselectivity of the reactions. The alcohol adducts thus obtained are related to a biologically important family of polyketide natural products. Remarkably, the syn,syn and syn,anti aldehyde substrates were converted to the stereopentad adducts with virtually complete diastereoselectivity and no discernable mismatching (equations 26 and 27). The major byproducts, produced in <10% yield, resulted from partial racemization of the allenylpalladium and/or allenylzinc intermediates. As expected, all the additions proceed with overall inversion at the propargylic stereocenter. 

This reaction is based on the formation of a blue color on oxidation of alcohols with potassium dichromate and sulfuric acid. The reaction is also positive with other easily oxidizable organic compounds. However, if nitric acid is used instead of sulfuric acid, the characteristic blue-violet color is given in the cold only by compounds containing the OH group (34). This reaction has been used for detecting small amounts of alcohols in esters, ethers, ketones, and biological and technical liquids (35, 36). 

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