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The development of simple, efficient, environmentally benign and economically
viable chemical processes or methodologies for widely used organic compounds is in great
demand (Meshram and Patil, 2009).
Acetals, dithioacetals and oxathioacetals are the most commonly used protecting
groups for aldehydes (Bandgar and Bettigeri, 2004) and ketones in the multi-step synthesis
of many natural products (Green and Wuts, 1991; and Kocienski, 1994). Among the
various protecting groups, oxathioacetals and dithioacetals are versatile (Corey and Seebach,
1965 and 1966; Lynch and Eliel, 1984; and Utimoto et al., 1990) because of their ease of
formation/removal and also their stability under a variety of reaction conditions. In addition
to carbonyl protection, they behave as masked acyl anions (Seebach, 1969; Grobel and
Seebach, 1977; Eliel and Morris, 1984; and Page et al., 1989) or masked methylene functions
(Pettit and Van Tamelen, 1962) in carbon-carbon bond-forming reactions.
Generally, oxathioacetals are prepared by condensation of carbonyl compounds with
2-mercaptoethanol using strong protic or Lewis acids (Ralls et al., 1949; and Djerassi and Gorman, 1953) as catalysts, but these procedures are often accompanied by long
reaction times, unsatisfactory yields and the use of stoichiometric amounts of catalysts. Even
though silicon reagents such as trimethylsilyl triflate (Wilson et al., 1968; Burczyk and Kortylewiez, 1982; Yadav and Fallis, 1988; Cputo et al., 1997; and Karimi and Seradj, 2000)
and triisopropylsilyl triflate (Ravindranathan et
al., 1995; and Steinz et al., 1997),
perchloric acid (Mandal et al., 2002),
LiBF4 (Yadav et al., 2001), organic ammonium
tribromide (Mandal et al., 2001), and zirconium tetrachloride (Karimi and Seradj, 2000) are found
to be efficient catalysts for this conversion, there are still some limitations, including
expensive catalysts, strongly acidic conditions and moderate yields of products. |
