Page 354 - Advanced Organic Chemistry Part B - Reactions & Synthesis

326 In the corresponding E-alkene, where this factor is not present, the cyclization is much
less stereoselective. A stabilizing interaction between the siloxy oxygen and the Hg 2+
CHAPTER 4 center has also been suggested. 108
Electrophilic Additions Reaction of Hg O CCF or Hg O SCF with a series of dibenzylcarbinols gave
to Carbon-Carbon 2 3 2 3 3 2 109
Multiple Bonds exo cyclization for formation of five-, six-, and seven-, but not eight-membered rings.
1) Hg(O CCF )
OH 2 3 2 ring exo:
or Hg(O SCF ) PhCH 2 O CH HgCl n endo
3
3 2
) CCH (CH ) CH 2 size
(PhCH 2 2 2 2 n CH 2 PhCH
2) NaCl 2 ( )n 1 5 > 99:1
2 6 >99:1
3 7 >99:1
4 8 -
Benzyl carbamates have been used to form both five- and six-membered nitrogen-
containing rings. The selectivity for N over O nucleophilicity in these cases is the
result of the nitrogen being able to form a better ring size (5 or 6 versus 7 or 8) than
the carbonyl oxygen.
NHCO CH Ph 1) Hg(OAc) 2
2
2
2) NaBH
CH 3 4 CH 3 N CH 3
2
CO 2 CH Ph 86% Ref. 110
CH 3
1) Hg(O 2 CCF )
3 2
NHCO CH Ph
2
2
2) KBr CH 3 N CH HgBr
2
CO CH Ph 98%
2
2
Ref. 111
The trapping of the radical intermediate in demercuration by oxygen can be exploited
as a method for introduction of a hydroxy substituent (see p. 295). The example below
and Entries 3 and 4 in Scheme 4.6 illustrate this reaction.
NHCO CH Ph O NCO CH Ph
OCH 2 2 2 2 2
O NCO CH Ph O 2
2
2
)
CH CH 1) Hg(NO 3 2 CH 3 CH OH
3
2
CH HgBr
CH 3 2 NaBH
CH CH 2 CH 2) KBr 4 80%
2
Ref. 112
Cyclization induced by mercuric ion is often used in multistep syntheses to form
five- and six-membered hetereocyclic rings, as illustrated in Scheme 4.6. The reactions
in Entries 1 to 3 involve acyclic reactants that cyclize to give 5-exo products. Entry 4
is an 6-exo cyclization. In Entries 1 and 2, the mercury is removed reductively, but in
Entries 3 and 4 a hydroxy group is introduced in the presence of oxygen. Inclusion of
triethylboron in the reduction has been found to improve yields (Entry 1). 113
108
A. Garavelas, I. Mavropoulos, P. Permutter, and G. Westman, Tetrahedron Lett., 36, 463 (1995).
109 H. Imagawa, T. Shigaraki, T. Suzuki, H. Takao, H. Yamada, T. Sugihara, and M. Nishizawa, Chem.
Pharm. Bull., 46, 1341 (1998).
110
T. Yamakazi, R. Gimi, and J. T. Welch, Synlett, 573 (1991).
111
H. Takahata, H. Bandoh, and T. Momose, Tetrahedron, 49, 11205 (1993).
112 K. E. Harding, T. H. Marman, and D.-H. Nam, Tetrahedron Lett., 29, 1627 (1988).
113
S. H. Kang, J. H. Lee, and S. B. Lee, Tetrahedron Lett., 39, 59 (1998).