Pyridinium acetate lactonization with

Mitsunobu reaction as well as by mesylation and subsequent base treatment failed, the secondary alcohol was inverted by oxidation with pyridinium dichromate and successive reduction with sodium borohydride. The inverted alcohol 454 was protected as an acetate and the acetonide was removed by acid treatment to enable conformational flexibility. Persilylation of triol 455 was succeeded by acetate cleavage with guanidine. Alcohol 456 was deprotonated to assist lactonization. Mild and short treatment with aqueous hydrogen fluoride allowed selective cleavage of the secondary silyl ether. Dehydration of the alcohol 457 was achieved by Tshugaejf vesLCtion. The final steps toward corianin (21) were deprotection of the tertiary alcohols of 458 and epoxidation with peracid. This alternative corianin synthesis needed 34 steps in 0.13% overall yield. 

Alternatively, the mesylate 77 was treated with sodium iodide in refluxing 2-butanone to produce the olefin 90 (Scheme 3). " Oxymercuration-demercuration reaction of 90 using mercnric acetate-sodinm borohydride afforded a 1 1 mixture of 91 and 92. Compound 92 was mesylated, rednced and cyclized in boiling ethanol, in the presence of sodium acetate, to give 93. This was converted by standard steps into 94, and subsequent hydrolysis of 94 followed by oxidation with pyridinium chlorochromate afforded the lactone 95 (28% overall yield from 92), which was nsed in the synthesis of retronecine (22). " ... 

On treatment with a non-enolizable aldehyde and sodium hydride, rruns-related a-hydroxyepoxides were converted to acetals. The reaction is thought to proceed by way of a hemiacetal salt, as illustrated in Scheme 2. L-Galactono-1,4-lactone, a by-product of the sugar industry, furnished a diastereomeric mixture of 5,6-alkylidene acetals 18 in >60% yield on microwave irradiation in the presence of long-chain aldehydes and montmorillonite KSF as catalyst, and 4,6-O-alkylidene derivatives 19 of D-glucose have been prepared by use of long-chain aldehydes and catalytic pyridinium />-toluenesulfonate in benzene under azeotropic removal of water. Compounds 18 and 19 were of interest in liquid crystal studies. 

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