Page 472 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Another particularly reactive form of titanium is generated by including 0.25 equivalent 445
of I . This reagent permits low-temperature reductive deoxygenation to alkenes. 241
2
SECTION 5.6
O CH 3 CH 3 Dissolving-Metal
TiCl 3 Reductions
CH 3 3.3. eq Li
0.25 eq I 2
93%
64:36 E:Z

Titanium metal is also activated by TMS-Cl. 242 These conditions were used in a number
of dimerizations and cyclizations, including the formation of a 36-membered ring.

O O Ti Ph
(CH ) (CH )
2 26
Ph Ph TMS-Cl 2 26
Ph
90%

Another process that is widely used involves reduction by Zn-Cu couple. This
reagent is especially reliable when prepared from TiCl purified as a DME complex, 243
3
and is capable of forming normal, medium, and large rings with comparable efficiency.

TiCl 2
O CH(CH 2 12 O
) CH
Zn-Cu 80%
9:1 E:Z

Ref. 244

The macrocyclization has proven useful in the formation of a number of natural
products. 245 These conditions have been used to prepare 36- and 72-membered rings.

H
CH OCH Ph
O CH O 2 2 TiCl 3
Zn/Cu
O
O CH
H
OCH Ph
O CH 2 2
O
56%
Ref. 246

241
S. Talukadar, S. K. Nayak, and A. Banerji, J. Org. Chem., 63, 4925 (1998).
242 A. Furstner and A. Hupperts, J. Am. Chem. Soc., 117, 4468 (1995).
243
J. E. McMurry, T. Lectka, and J. G. Rico, J. Org. Chem., 54, 3748 (1989).
244 J. E. McMurry, J. R. Matz, K. L. Kees, and P. A. Bock, Tetrahedron Lett., 23, 1777 (1982).
245 J. E. McMurry, J. G. Rico, and Y. Shih, Tetrahedron Lett., 30, 1173 (1989); J. E. McMurry and
R. G. Dushin, J. Am. Chem. Soc., 112, 6942 (1990).
246
T. Eguchi, K. Arakawa, T. Terachi, and K. Kakinuma, J. Org. Chem., 62, 1924 (1997).