Tin-Mediated Regioselective Benzylation and Allylation of Polyols: Applicability of a Catalytic Approach Under Solvent-Free Conditions

Abstract

The first catalytic version of the stannylene-mediated benzylation and allylation of polyols is reported. The methodology is based on a simple solvent-free protocol that significantly advances, in terms of both experimental ease and synthetic scope, the applicability of tin-promoted selective protections. The described approach is indeed endowed with a very large number of advantages over routine protocols: use of a low catalytic loading of cheap Bu 2 SnO, a single-step process with avoided use of solvents, a minimally demanding experimental procedure with reactions performed under air, reduced reaction times, a much simpler work up, a wide target scope, and yields that, in many cases, compare favorably to routine protocols. In addition, the catalytic solvent-free approach extends the scope of stannylene chemistry to unprecedented applications to reducing sugars and in the synthesis of highly benzylated building blocks otherwise accessed through much more demanding procedures. From a conceptual point of view, the described results indicate that solvent-free conditions can assist the development of catalytic approaches otherwise ineffective in solution.

Synthesis of brassinosteroids with a keto group in the side chain

“…However, no reaction was observed under these conditions with the diol 9 (Scheme 6). Recently, a simple solvent-free procedure of the stannylene-mediated benzylation of polyols was reported [26]. The same protocol, when applied to the diol 8, gave a 1:1.5 mixture of benzyl ethers 17 and 18 in a good yield.…”

Orthogonal protection of saccharide polyols through solvent-free one-pot sequences based on regioselective silylations

“…This effort led to a simple protocol for the selective benzylation of primary saccharide alcohols [34], the first catalytic tin-mediated procedure for regioselective benzylation/allylation of hydroxy groups incorporated into vicinal diols [35], and three alternative acetalation protocols [36]. Besides the avoided use of solvents, these approaches appear advantageous owing to their experimental ease, the reactions being performed under air by simply mixing the requisite reagents.…”

Regioselective mono and multiple alkylation of diols and polyols catalyzed by organotin and its applications on the synthesis of value-added carbohydrate intermediates

“…6,7 However, the organotin compounds have to be either minimized in its usage to a catalytic amount, 8e12 or totally abandoned 13e19 now due to their potential inherent toxicity. 20,21 The regioselective alkylation of carbohydrates and polyols using a catalytic amount of organotin have been reported by two other groups 22,23 as well as our group 24 recently. Our developed method used 0.1 equiv of TBAB (tetrabutylammonium bromide) and 0.1 equiv of organotin as catalyst, and used excess amount of potassium carbonate to deprotonate the hydroxyl group and push through the reaction (Fig.…”

“…c o m/ l o ca t e / t e t of 0.1 equiv of TBAB and 1.1 equiv of K 2 CO 3 , indicating the key role of dibutylstannylene acetal in the reaction mechanism. In Alfonso et al's work, 22 0.1 equiv of dibutyltin oxide was also used, but in conjuncture with 0.3 equiv of TBAB, excess amount of DIPEA (diisopropylethyl amine) and alkylation reagents (Fig. 1b).…”