REDUCTIVE CLEAVAGE OF VINYL PHOSPHATES

REDUCTIVE CLEAVAGE OF VINYL PHOSPHATES PREPARATION OF 17 i-tert-BUTOXY-5a-ANDROST-2-ENE... 

A practical synthesis of 1,3-OX AZEPINES VIA PHOTOISOMERIZATION OF HETERO AROMATIC V-OXIDES is illustrated for 3,1-BENZOXAZEPINE. A hydroboration procedure for the synthesis of PERHYDRO-9b-BORAPHENALENE AND PERHYDRO-9b-PHEN-ALENOL illustrates beautifully the power of this methodology in the construction of polycyclic substances. The conversion of LIMONENE TO p-MENTH-8-EN-YL METHYL ETHER demonstrates a regio-and chemoselective method for the PHOTOPROTONATION OF CYCLOALKENES. An efficient method for the conversion of a ketone to an olefin involves REDUCTIVE CLEAVAGE OF VINYL PHOSPHATES. A mild method for the conversion of a ketone into the corresponding trimethylsiloxy enol ether using trimethylsilyl acetate is shownforthe synthesis of (Z)-3-TRIMETHYLSILOXY-2-PENTENE. 

REDUCTIVE CLEAVAGE OF VINYL PHOSPHATES, 61, 116 REDUCTIVE COUPLING. 60, 113 table, 60, 116... 

Appropriate quenching of a reductively formed lithium enolate with a carboxylic acid anhydride, chloride, methyl chloroformate or diethyl phosphorochloridate yields the corresponding enol esters, enol carbonates or enol phosphates. These derivatives may be transformed into specific alkenes via reductive cleavage of the vinyl oxygen function, as illustrated by the example in Scheme 8. 

Initial, electrochemical reduction of /S-dicarbonyl enol phosphates linked to an ole-finic chain gives cleavage of the phosphate with formation of a vinylic radical [222-224]. Reduction in DMF takes place in the potential range —2.0 to —2.3 V and may lead to bicyclic products as illustrated in Scheme 23. 

The cleavage of carbon-oxygen bonds from alkenyl or aryl phosphates can be accomplished under reductive conditions with a low valent metal. As vinyl phosphates can be formed readily from ketones, this procedure provides a method to convert a ketone to an alkene. For example, the alkenyl phosphate 74 was prepared by trapping the enolate formed on reduction of the enone 73 and was converted into the alkene 75 (7.55). The chemistry therefore provides a method to prepare structurally specihc alkenes. Low-valent titanium (prepared for example by reduction of titanium(III) chloride with potassium metal) is a convenient alternative to lithium or sodium in liquid ammonia or an amine for the reductive cleavage of alkenyl or aryl phosphates. 

Despite these modifications, Ireland s original method of both vinyl phosphate formation and reduction remains the most popular. In studies on the cationic rearrangements of [4.3.2]propellanes, Smith et converted a tricyclic ketone, via the diethyl vinylphosphate derivative, into the corresponding alkene (Scheme 26). In a similar manner, Kamata et al prepared A - and A -steroids (Scheme 27) with excellent control of regiochemistry. As shown in this example, esters are susceptible to cleavage under the standard reduction conditions (Li, NHa, Bu OH, -35 C), while acetals survive. 

For preparative purposes, titanium metal can be used in place of sodium or lithium in liquid ammonia for both the vinyl phosphate and aryl phosphate" cleavages. The titanium metal is generated in situ from TiCla by reduction with potassium metal in tetrahydrofuran. 

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