Alcohols, oxidizing reagents allenes

Selective oxidation of secondary alcohols to ketones is usually performed with CrOj/HjSO, I I in acetone (Jones reagent) or with CrOjPyj (Collin s reagent) in the presence of acid-sensitive groups (H.G. Bosche, 1975 C. Djerassi, 1956 W.S. Allen, 1954). As mentioned above, a,)S-unsaturated secondary alcohols are selectively oxidized by MnOj (D.G. Lee, 1969 D. Arndt, 1975) or by DDQ (D. Walker, 1967 H.H. Stechl, 1975). 

An alternative, but related, route to allenic titanium reagents from propargylic esters has been reported recently. Reaction of titanocene dichloride with BuMgCl and Mg yields a reactive titanocene intermediate, formulated as Cp2Ti. This reduced Ti species reacts in situ by oxidative addition to propargylic acetates. The allenyltitanium reagents thus produced add to aldehydes and ketones, as expected, to afford homopropargylic alcohols (Table 9.27) [43]. 

Oxidation of 3-allenic alcohols of the type (1) with Payne s reagent results in formation of 7-lactones (2) probably via an allenic oxide (a). ... 

Compound 11 is, however, unexpectedly unreactive with Wittig-Horner reagents. Upon heating with the carbanion of ester phosphonates an addition across the allenic bond occurs [14]. In contrast, a slow normal 1,2-addition takes place [14] with the ylide from cyano-methylphosphonate but, unexpectedly, this proceeds with concomitant inversion at the chiral axis as shown in Scheme 3, to give a mixture of 6R or 6S, and (9E)- or (9Z)-isomers 12-15. However, a fast and very clean 1,2-addition occurs with the ethynyl ketone 18 to yield the esters 19 and 20 (Scheme 4). DIB AH reduction of the separated stereoisomers gives the allenic alcohols 21 and 22 in high yield. Mild oxidation to the aldehydes 23 and 24, followed by their condensation with the acetylenic Cio-bis-ylide 25, leads to the stereoisomeric 15,15 -didehydromimulaxanthins 26 and 28, respectively (Schemes 5 and 6). The optically active. 

Further investigations of oxidations by dimethyl sulphoxide activated by oxalyl chloride have shown the system to be of general use, although allenic and acetylenic alcohols are not oxidized. The same reagent activated by dicyclo-hexylcarbodi-imide has been reported to yield ketones from alcohols in the presence of dithianes/°... 

Allylic anda-Allenic Alcohols. The electrochemical generation of a selenenylat-ing reagent, presumably PhSeOR, from a catalytic amount of diphenyl diselenide in the presence of water or an alcohol (ROH) is the basis of a new method for direct conversion of alkenes to allylic alcohols or ethers (Scheme 13). Regio-specific Markovnikov oxyselenenylation followed by electrochemical oxidation and selenoxide elimination, regenerating a selenenylating species, accounts for a sequence that is obviously closely related to the bromide-mediated method reported last year (5,158). 

From the chiral epoxide 56, the TMM cycloaddition precursor 55 for construction of the tetraquinane stmcture was prepared in a straightforward manner. Oxidation of epoxy alcohol 56 by the Swem protocol followed by treatment with Bestmann-Ohira reagent (E) produced alkyne 70. Iron-catalyzed Sisr2 -type reaction of 70 afforded an allene moiety as a diastereo-meric mixture in a 1 1 ratio and this mixture was used in the next step without the separation of isomers. Allenyl alcohol 71 was protected as a TBDPS ether to give 0-silylated allene 72. Deprotection of the acetal of 72 was delicate as the allenyl moiety was not stable under acidic condition. Fortunately, treatment of 72 with p-toluenesulfonic acid monohydrate in presence of formaldehyde afforded aldehyde 73, and subsequent treatment with p-toluene sulfonyUiyrazide furnished the precursor 55 for the tandem cycloaddition reaction (Scheme 21). 

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