Tributyltin hydride thionocarbonates

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 tributyltin hydride reduction usually proceeds without complications. The most common byproduct is starting alcohol, which is postulated to be derived from a mixed thioacetal. Use of the phenyl thionocarbonate ester has been reported to minimize this side reaction in cases where it is a problem.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... 

A very useful radical-based reaction has been developed that can be applied to alcohols. As seen in chapter 4, conversion of an alcohol such as 73 to a thionocarbonate (74), followed by treatment with tributyltin hydride under radical conditions, gives cleavage to the C—O bond to give, in this case, 75. This example is take from Beak s synthesis of (-)-dihydropinidine, and is an example of a transformation called the Barton deoxygenation or the Barton-McCombie reaction. ... 

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). 

Oxime ethers derived from hydroxy aldehydes, upon conversion to their phenyl thionocarbonate esters, undergo radical cyclizations resulting in the formation of carbocycles. For example, an oxime ether obtained from D-glucose is converted into its phenyl thionocarbonate ester at C-5 and, upon heating in benzene in the presence of tributyltin hydride, affords cyclopentanes in 93% yield as a 62 38 mixture of two diastereomers (eq 7). In general, only low to modest stereoselectivity between the newly formed stereocenters is observed in a number of substrates examined. 

The alternative route to (+)-swainsonine by Chen and Tsai used a very similar strategy, but instead formed the indolizidine system by making the C-8/C-8a bond (Scheme 87). In this case the amine 647 and commercially available 2,3-isopropyhdene-D-ribono-1,4-lactone (648) were condensed to form an amide, the terminal diol unit of which was cleaved with lead tetraacetate to give the 5-hydroxypyrroHdinone (—)-649 after acid-promoted cyclization. The thionocarbonate derivative (—)-650 was then treated with iodobenzene bis(trifluoroacetate) to deprotect the acylsi-lane. Intermediate (—)-651 then underwent radical-induced cyclization with tributyltin hydride to give the 8-hydroxyindolizidin-3-one 652 in... 

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