Page 519 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Cis- and trans-bis-benzenesulfonylethene are also acetylene equivalents. The two 493
sulfonyl groups undergo reductive elimination on reaction with sodium amalgam.
SECTION 6.1
SO Ph Diels-Alder Reactions
PhSO 2 2 Na Hg
+ C C
H H SO 2 Ph MeOH
SO 2 Ph 69%
Ref. 70

Vinylphosphonium salts are reactive as dienophiles as a result of the EWG
character of the phosphonium substituent. The D-A adducts can be deprotonated to
give ylides that undergo the Wittig reaction to introduce an exocyclic double bond.
This sequence of reactions corresponds to a D-A reaction employing allene as the
dienophile. 71

+
+
+H C CHPPh 3 PPh 3 1) LiNR 2 CH 2
2
O
2) CH 2
96% 50%
The use of 2-vinyldioxolane, the ethylene glycol acetal of acrolein, as a dienophile
illustrates application of the masked functionality concept in a different way. The
acetal itself would not be expected to be a reactive dienophile, but in the presence of a
catalytic amount of acid the acetal is in equilibrium with the electrophilic oxonium ion.
O +
CH 2 CH +H + CH 2 CH CH O CH CH OH
2
2
O
Diels-Alder addition occurs through this cationic intermediate at room temperature. 72
Similar reactions occur with substituted alkenyldioxolanes.
2 mol % 2
O R 1 CF SO H R
+ 3 3
O CH CHR 2 –78 –10°C R 1
O O

This reaction has been used to construct the carbon skeleton found in dysidiolide, a
73
cell cycle inhibitor isolated from a marine sponge. In this case, the reactive oxonium
ion intermediate was generated by O-silylation.
CH 3
O CH
+ 3 CH 3
TMSOTf
CH 3 O CH 3
CH 3
OTBDPS H
TBDPSO O
O
70 O. DeLucchi, V. Lucchini, L. Pasquato, and G. Modena, J. Org. Chem., 49, 596 (1984).
71
R. Bonjouklian and R. A. Ruden, J. Org. Chem., 42, 4095 (1977).
72 P. G. Gassman, D. A. Singleton, J. J. Wilwerding, and S. P. Chavan, J. Am. Chem. Soc., 109, 2182
(1987).
73
S. R. Magnuson, L. Sepp-Lorenzino, N. Rosen, and S. J. Danishefsky, J. Am. Chem. Soc., 120, 1615
(1998).