Carbonyl reduction using other methods

The reductive coupling of carbonyl compounds to pinacols (i.e., 1,2-diols) is usually performed in the presence of a low-valent metal such as Li(0), Sm(II) or Ti(III). Under 

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There has been recent interest in naphtho-fused dithiepines as chiral acyl anion equivalents, particularly since the starting dithiol 128 can be obtained in enan-tiomerically pure form (89TL2575). This is transformed using standard methods into the dithiepine 129, but showed only moderate diastereoselectivity in its addition to carbonyl compounds. On the other hand, as we have seen previously for other systems, formation of the 2-acyl compound 130 and reduction or addition of a Grignard reagent gave the products 131 with much better stereoselectivity (91JOC4467). 

More recently, methods based on the use of mild reductants, able to transfer a single electron to the polyhaloalkyl halide, have been described. Various metals or their derivatives have been employed ruthenium, platinum and their complexes in low oxidation state, iron" and its carbonyl complexes, or tetrakis(triphenylphosphane)palladium. Sodium arcncsul-finate, sodium dithionite" and various oxidants have also been used. Other examples of polyhaloalkyl halide additions to simple alkenes are summarized in Table 1. Typical examples are the formation of diiodide 6, chloro iodide 7, and iodo steroid 8. ... 

Gribble and Saulnier (79) have extended their ellipticine synthesis 43) to the synthesis of 9-methoxyellipticine (2) (Scheme 24). One of the key features of this approach is the regioselective nucleophilic addition to the C-4 carbonyl group of pyridine anhydride 28. The other noteworthy transformation is the conversion of keto lactam 142 to the diol 143 with methyllithium, a process that presumably involves cleavage of the initial adduct to a methyl ketone which undergoes cyclization at the C-3 position of the indolyl anion. Reduction of 143 with sodium borohydride completes the synthesis of 2, in 47% overall yield from 5-methoxyindole (139). Gribble and students 80) have also used this method to synthesize 8-methoxyellipticine (134), 9-fluoroellipticine (144), and the previously unknown 7,8,9,10-tetrafluorellipticine (145), each from the appropriate indole. 

The acid-catalyzed cleavage of the tert-butoxycarbonyl group Is the best method to form the parent benzazepinone. Other methods used have been the Pd/C hydrogenolysis of a benzyloxycarbonyl group,and zinc mediated reductive cleavage of trichloroethoxy and trichloro-tert-butoxy carbonyl groups. ... 

Deoxy-l,2 5,6-di-0-isopropylidene-a-D-ribo-hexofuranose has been prepared by a variety of other methods, the most widely used of which is the Raney nickel reduction of the 3-S-[(methylthio)carbonyl]-3-thioglucofuri nose... 

Similarly, nitro groups are not so easily reduced as with lithium aluminium hydride itself, and 4-nitrobenzaldehyde can be converted into 4-nitrobenzyl alcohol in 75% yield. Aldehydes and ketones are reduced to alcohols and there is no advantage in the use of mixed hydrides in these cases (although the stereoselectivity may vary if reduction leads to diastereomeric alcohols). With diaryl ketones and with aryl alkyl ketones, however, the carbonyl group is reduced to the methylene group, and this procedure offers a useful alternative to the Clemmensen or other methods for reduction of this type of ketone. 

The synthetic utility of the elimination reaction is enhanced by its usefulness in preparing a number of compounds difficult or impossible to synthesize by other methods. For example, in the preparation of RMn(CO)s compounds by the reaction of XMn(CO)s with anionic alkylating agents, reduction of XMn(CO)s to Mn2(CO)io lowers the yields (63). The alternative route of reacting the metal carbonyl anion with an acid chloride and then decarbonylating the resulting acyl complex is often more desirable (55, 64). 

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