Page 479 - Advanced Organic Chemistry Part B - Reactions & Synthesis

452 Scheme 5.13. (Continued)

CHAPTER 5 a. E. J. Corey and R. L. Carney, J. Am. Chem. Soc., 93, 7318 (1971).
b. E. J. Corey, R. L. Danheiser, and S. Chandrasekaran, J. Org. Chem., 41, 260 (1976).
Reduction of c. J. E. McMurry and R. G. Dushin, J. Am. Chem. Soc., 112, 6942 (1990).
Carbon-Carbon Multiple d. D. R. Williams and R. W. Heidebrecht, Jr., J. Am. Chem. Soc., 125, 1843 (2003).
Bonds, Carbonyl e. M. Nazare and H. Waldmann, Chem. Eur. J., 7, 3363 (2001).
Groups, and Other f. J. E. McMurry, M. P. Fleming, K. L. Kees, and L. R. Krepski, J. Org. Chem., 43, 3255 (1978).
Functional Groups g. C. B. Jackson and G. Pattenden, Tetrahedron Lett., 26, 3393 (1985).
h. N. L. Allinger, Org. Synth., IV, 340 (1963).
i. J. J. Bloomfield and J. M. Nelke, Org. Synth., 57, 1 (1977).
j. M. Makosza and K. Grela, Synlett, 267 (1997).

trap the pinacol as a cyclic siloxane. The reaction in Entry 2 is thought to involve
Ti(II) as the active reductant and to proceed by a mechanism of the type described on
p. 447. These conditions were also successful for the reaction shown in Entry 1. Entry
3 involves formation of a 14-membered ring using a low-valent titanium reagent. The
product is a mixture of all four possible diastereomeric diols in yields ranging from
7 to 21%. Entry 4 is an example of a pinacol reduction using a vanadium reagent
prepared in situ from VCl and Zn, which tends to give a high proportion of cis-diol as
3
a result of chelation with vanadium. Entry 5 shows the synthesis of a sensitive polyun-
saturated lactam. The cis-diol was formed in 60% yield. In this particular case, various
low-valent titanium reagents were unsuccessful. Entries 6 and 7 describe conditions
that lead to alkene formation. Entries 8 to 10 are acyloin condensations. The reaction
in Entry 8 illustrates the classical conditions. Entry 9 is an example of the reaction
conducted in the presence of TMS-Cl to trap the enediolate intermediate and make the
reaction applicable to formation of a four-membered ring. The example in Entry 10
uses sodium in the form of a solid deposit on an inert material. This is an alternative to
the procedures that require dispersion of molten sodium in the reaction vessel (Entries
8 and 9).

5.7. Reductive Deoxygenation of Carbonyl Groups

Several methods are available for reductive removal of carbonyl groups from
organic compounds. Reduction to methylene groups or conversion to alkenes can be
achieved.
O
R′
R
R′ R′
R R

5.7.1. Reductive Deoxygenation of Carbonyl Groups to Methylene
Zinc and hydrochloric acid form a classical reagent combination for conversion
of carbonyl groups to methylene groups, a reaction known as the Clemmensen
reduction. 272 The corresponding alcohols are not reduced under the conditions of the

272
E. Vedejs, Org. React., 22, 401 (1975).