Thionocarbonates reduction with tributyltin hydride

Methylation of avermectins B and B2 leads to the corresponding derivatives of the A series (49). A procedure involving the oxidation of the 5-methoxy group with mercuric acetate and NaBH reduction of the 5-keto-intermediate allows the conversion of the A to the B components (50). The 23-hydroxy group of the "2" components, after selective protection of the other secondary hydroxy groups, is converted to a thionocarbonate, which can be elirninated to give the 22,23-double bond of the "1" components alternatively it can be reduced with tributyltin hydride to the 22,23-dihydro derivatives (= ivermectins) (51). 

The second starting material, mesylate 426, was synthesized from alcohol 422. Swern oxidation followed by addition of propargyl zinc bromide led to compound 423 in 79% yield, which was converted into thionocarbonate 424 (41% yield over three steps). Radical ring closure with tributyltin hydride and oxidation with Dess-Martin periodinane afforded ketone 425. Treatment with DMAP led to the rearrangement of the double bond and subsequent reduction with sodium borohydride and mesylation gave compotmd 426 (Scheme 8.6). 

A variation of the tributyltin hydride reduction is available that can be applied to alcohols. The alcohol is converted to a thionocarbonate and then reduced with BusSnH and AIBN, which circumvents the need for... 

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