Furans 2.5- adduct from bromination

In this case the trityl-protected lactaldehyde 882 is prepared from trityl lactate 463 by reduction of the ester with lithium aluminum hydride followed by oxidation under Swem conditions. An aldol-type addition of titanated furan produces a 6 1 mixture of anti and syn adducts from which the pure anti diastereomer 883 is isolated by column chromatography (55% yield, >95% de). Treatment of 883 with bromine in methanol followed by acidic hydrolysis gives the ulose 885. A 1,4-reduction to 886, acetylation to 887, and ozonolysis affords the unstable aldehyde 888, which is immediately coupled with 2,5,6-triamino-4-pyrimidinol (889) to afford 890 [237]. 

The realization of 99 was not trivial. The five-step route (Scheme 22) commenced from the cycloaddition of 103 with furan. The resulting adduct 104 was reduced by low-valent titanium to 105, which was brominated to afford 106. Dehydrobromi-nation of 106 in the presence of 101 provided endoxide 107. Again, low-valent... 

Bromination of pyrrole with bromine gives mainly the thermodynamically more stable 3-bromo derivative, which is produced from the 2-isomer, formed initially under kinetic control, through isomerization catalyzed by the reaction product HBr. If N-bromosuccinimide is used for bromination, no HBr is produced and the 2-isomer dominates (8IJOC222I). Bromination of furan gave cis- and frans-2,5,-dibromo-2,5-dihydrofurans (isolated as the dimethoxy derivatives) in a ratio of 3 I (67BCJ130). A spectroscopic study carried out at low temperature (- 50°C) confirmed this ratio and also showed that 20% of trans-2,3-dibro-mo-2,3-dihydrofuran was formed (75CC875). It is not known whether these adducts (which demonstrate the competition between 1,4- and 1,2-addition) are intermediates along the pathway for bromine substitution, or whether they are produced in a separate equilibrium as is the case for bromination of benzo[6]furan. 

The former assignment of the structures of the cyclobutadiene adducts of hexakis-(trifluoromethyl)benzvalene by Warrener was corrected by the X ray analysis of the bromination product of the exo-adduct98). The reaction of the adduct, formed from hexakis(trifluoromethyl)benzvalene and furan, with bromine yields a ew-adduct, probably because of the high steric requirement of the trifluoromethyl groups on the bridge-head. 

Benzo[b]furan was first prepared from coumarin by addition of bromine and dehydrobromination of the adduct 7 with KOH ... 

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