Cyclization boron trifluoride-mediated

Methoxy-2-alkylthiothiazolines (327) are obtained by thermal or diethyl ether-boron trifluoride mediated intramolecular cyclization of the intermediate dithiocarbamate (326) which is prepared... 

The electron-rich thiophene ring system can be elaborated into complex, fused thiophenes by acid-mediated intramolecular annelation reactions. For example, treatment of alcohol 96 with trimethylsilyl triflate promoted a Friedel-Crafts acylation and subsequent dehydration giving benzo[b]thiophene 97, a potential analgesic <00JMC765>. Treatment of ketone 98 with p-toluenesulfonic acid resulted in the formation of fused benzo[b]thiophene 99 <00T8153>. Another variant involved the cyclization of epoxide 100 to fused benzo[f>]thiophene 101 mediated by boron trifluoride-etherate . 

Boron trifluoride etherate-mediated cyclization (Equation 24) of the pyran derivative 62 (prepared from D-glucose) is used in the synthesis of AB ring system of ciguatoxine <1997T3057>. 

Resin-based chemistry has been used to construct 480 from 481 by use of boron trifluoride to mediate both the cyclization and cleavage steps <1999AGE1121 >. When the monosaccharide was bound to the polystyrene resin (Merrifield and MPP type) by an alkylsulfonyl linker 482 and cyclization was mediated by 2- r/-butylimino-2-diethylamino-l,3-dimethylperhydro-l,3,2-diazaphosphorine, the reaction had low stereoselectivity and the products included tricyclic oxetanes and oxiranes <2004EJ04177>. The Mitsunobu reaction was used to obtain intramolecular N-alkylation of 483 and formation of 484 <2005AGE3732>. 

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