Cyanohydrin 3 + 2 cyclisation

Among a host of other phosphine-catalysed reactions in which the initial step is the formation of a reactive phosphoniobetaine intermediate by addition to a carbon-carbon double or triple bond are intramolecular cyclisations leading to benzobicyclo[4,3,0]-compounds, " cyclic ethers " and lactones,and a great many intermolecular reactions, e.g., a [3 -b 3]-annulation of modified t-butyl allylic carbonates and alkylidenemalonitriles to give cyclohexenes,phosphine- (and fluoride)- catalysed routes to 1,4-benzothiazepines from cyclic sulfenamides and alkynes, a [4- -3]-annu-lation of allylic carbonates with methyl coumalate to give functionalised bicyclo[3.2.2]nonadienes, the a-carbon addition of cyanide ion, generated in situ from cyanohydrins, to activated alkynes, and a stereoselective... 

This acid is derived from retusamine (Table3Pi), and has been synthesised by Kiyooka etal. 162) as shown in Scheme 18. The starting material (55) was the same as that used in the synthesis of a- and ji-retusanecic acids cf. Scheme 17), and a similar strategy was employed, except that ethylation of the starting material (55) preceded stereospecific cyanohydrin formation and cyclisation to give lactone (66). The absolute configuration of the resolved lactone acid, which was identical with retusaminic acid (67), was already known from the X-ray structure analysis of retusamine carried out by Wunderlich 319). 

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