ORGLIST:summary : problem with NBS

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From: Sehat Nauli (snauli$##$itsa.ucsf.edu)
Date: Sun May 25 2003 - 23:13:38 EDT

('binary' encoding is not supported, stored as-is) Hi,

This is a summary of answer to my question:
I used NBS in a bromination rx, however the yield's very low (4 day rx
time) and I noticed NBS got degraded over time as well. Any suggestions?

Answers:
1. the best conditions are 1.5 eq NBS, 0.1 eq AIBN in CCl4, 3-4 hours at
70-80 deg (check NMR frequently and stop just after appearance of bis-bromo).

2.use more NBS (up to 6 equivialents) in CCl4 in presence of benzoyl
peroxide and then in a second step a reaction with diethylhypophsphite
and ethyl-diisopropyl-amine in THF.
Ref: Synthesis 2001, No. 14

3. another procedure: sodium bromate/sodium hydrogensulphate in
water/organic solvent (exothermic rx).
Ref: Journal of Organic Chemistry
1998, 63, page 6023.

Thanks for the people who gave answers. I did increase my yield over 90%
by adding fresh benzoyl peroxide when it's consumed (after ~16 hours) and
also fresh NBS and doing the rx over 6 days. Additionally I monitored the
rx progress with NMR and this really helped in determining when to add
the appropriate fresh reagent.

********************************
Sehat Nauli

Department of Cellular and Molecular Pharmacology
Genentech Hall Room N542
UCSF Mission Bay
600 16th St.
San Francisco, CA 94107

Ph: 415-514-4071
********************************

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