Page 911 - Advanced Organic Chemistry Part B - Reactions & Synthesis

occurs in tandem with a carbonyl-ene reaction and results in both a ring closure and 887
ring expansion. 70
SECTION 10.1
R Reactions and
TMSO O R Rearrangement
SnCl 4
or OCH 3 Involving Carbocation
(CH ) Intermediates
2 n
2 n
CH(OCH ) TMSOTf, (CH )
3 2
H di-t-butyl-
pyridine H
R = CH , Ph
3
n = 1,2
These reactions appear to proceed through the sequence C → D → E . When the
seven-membered analog (n = 3) reacts, two products are formed. The more flexible
seven-membered ring accommodates the competing sequence. F → G → H.
O
R R R
(CH 2 ) n
OCH 3
(CH 2 ) n +
+
O CH 3 OCH 3 (CH 2 ) n
TMSO OTMS D E H
C
pinacol
carbonyl-ene
CH 3
(CH 2 ) n (CH 2 ) n + O OCH 3
+
O CH 3 OCH 3
R R
OTMS G OTMS H H
n = 3 only
F
The carbonyl-ene–pinacol sequence has also been observed in reactions leading to the
formation of tetrahydrofurans. 71

OH CH 3
CH 2
(CH 3 ) 2 C = O CH 2 O CH 3 +
CH 2 HO
HO
CH 3 + O
H CH 2 +
CH 3 OH O CH 3 O
CH 3
CH 3
O = CH CH 3
O
CH 2 CH 3
CH 3

The reaction has been developed for the synthesis of both oxygen heterocycles and
carbocyclic compounds. 72

70
S. Ando, K. P. Minor, and L. E. Overman, J. Org. Chem., 62, 6379 (1997).
71 P. Martinet and G. Moussel, Bull. Soc. Chim. Fr., 4093 (1971); C. M. Gasparski, P. M. Herrinton,
L. E. Overman, and J. P. Wolfe, Tetrahedron Lett., 41, 9431 (2000).
72
L. E. Overman, Acc. Chem. Res., 25, 352 (1992); L. E. Overman and L. D. Pennington, J. Org. Chem.,
68, 7143 (2003).