Meerwein -Ponndorf- Verley intramolecular

A tandem 1,4-addition-Meerwein-Ponndorf-Verley (MPV) reduction allows the reduction of a, /i-unsaturated ketones with excellent ee and in good yield using a camphor-based thiol as reductant.274 The 1,4-addition is reversible and the high ee stems from the subsequent 1,7-hydride shift the overall process is thus one of dynamic kinetic resolution. A crossover experiment demonstrated that the shift is intramolecular. Subsequent reductive desulfurization yielded fiilly saturated compounds in an impressive overall asymmetric reductive technique with apparently wide general applicability. 

S)-(+)-carvone. The key steps were Baeyer-Villiger oxidation, Oppenhauer oxidation, Meerwein-Ponndorf-Verley reduction, a stereospecific Grignard addition, and an intramolecular Sn2 reaction. 

This cleavage usually also provides, as a minor product, the ketone 3, formed by an intramolecular Meerwein-Ponndorf-Verley reduction and Oppenauer oxidation -Alkoxy ketones of this type (6) can be obtained as the major product by reduction of ketals (5) with diethylaluminum fluoride (1.2 equiv.) and pentafluorophenol (2.4 equiv.), (equation II). Note that the reduction is again effected with retention. 

In a different vein and as already pointed out in Chapter 8 (Scheme 8.6), the Meerwein-Ponndorf-Verley reduction is the reverse of the Oppenauer oxidation of aldehydes and ketones, and it is only a change of solvent that dictates whether the reaction that occurs is an oxidation or a reduction.The same catalyst is used. Scheme 9.19 is the reverse of Scheme 8.6. Thus, it is now suggested that the carbonyl oxygen of cyclohexen-3-one displaces an isopropoxy group (2-propoxy [ OCH(CH3)2]) from the catalyst, aluminum isopropoxide [Al(0-iPr)3].Then, after intramolecular hydride transfer, propanone (acetone, CH3COCH3) is lost by displacement from aluminum by the solvent, 2-propanol (isopropanol [CH3CH(OH)CH3]), and finally, the cyclo-... 

It is important to realise that chirality is a symmetry property of a whole molecule and cannot be localised in a particular centre or group (although it may be associated with the presence of a particular stereogenic unit). For this reason the intramolecular transfer of chirality is an impossibility. Transfer of chirality can only occur between two molecules and is quite uncommon. [An example is the enantioselective Meerwein-Ponndorf-Verley reduction of a ketone with a chiral alcohol.] What some authors refer to incorrectly as transfer of chirality is, in fact, retention of chirality in the course of modification of the stereogenic units present. 

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