Stereoselective Construction of N,O– and O,O‐Acetals by Pd‐catalyzed Addition of Heteroatoms to Alkoxyallene

Abstract

Mixed N,O-and O,O-acetals are key molecules in the synthetic organic chemistry found in many biologically important molecules represented in sugars and azasugars. 1 However, the more important values are found in their synthetic utilities as intermediates for the construction of iminium and oxocarbenium ions. 2 Most of their synthetic methods are based on the alkylation of iminium and oxocarbenium ions generated in situ from their precursors, with poor stereoselectivity in most cases. Recently, an eminent Korean synthetic organic chemist Young Ho Rhee succeeded a conceptually new method as stereoselective construction of N,O-and O,O-acetals by Pd-catalyzed addition of heteroatoms to alkoxyallene. 3 The p-toluenesulfonyl (Ts)-protected allylic amines and allylic alcohols (1a or 1b) were added to alkoxyallenes (2) in the presence of the chiral Pd-catalyst consisted of either L1 or L2 to yield N,O-and O,O-acetals (3a or 3b) with high stereoselectivity. This reaction has a wide scope of the alkyl substrates (R') shown in the Scheme 1. 3 In most cases the substrate cyclopropoxyallene was the best for the reaction of Ts-protected allylic amines and allylic alcohols with the chiral catalyst with either L1 or L2 in quantitative yield with more than 92% ee.The product bearing two olefins as allylic relationship with amine or oxygen as their centers afforded azacycles (4a) and oxacycles (4b) by ring-closing metathesis (RCM) with well-known Grubbs' catalyst. 4 In most cases catalytic amount of the first generation Grubbs' catalyst yielded the cyclic compound without any altering the stereocenters and functionalities to give α-alkoxy-pyrrolidine (I), piperidine (II), and furanose sugars (III and IV) (Figure 1). Furthermore, Os-catalyzed dihydroxylation was performed on the resultant olefinic bond in the cyclic compounds with all stereochemical information.This synthetic method i.e., Pd-catalyzed addition of alcohol followed by RCM enables to prepare apiose-containing oligosaccharides without assistance of directing and protecting groups. Polygalin G (8) as the high-lightened example of the apiose-containing trisaccharides was successfully achieved by initial combination of two units Scheme 1. Pd-catalyzed addition of heteroatoms to alkoxyallene. Figure 1. Synthesis of azacycles (I and II) and oxacycles (III and IV) as representive examples.Highlights 759 including allylic alcohol (5) and alkoxyallene (6) bearing monosaccharides at each side shown in Scheme 2. The coupling reaction was highly efficient as in 72% yield with high stereoselectivity (>20:1 dr). 5 The central ring in 7 was easily constructed by Ru-catalyzed RCM of the initial adduct between 5 and 6, which was dihydroxylated to give apiose-containing trisaccharides unit in polygalin G (8) in 93% yield.Further development enabled us to access synthetically challenging 2,3,6-trideoxypyranoglycosides including kigamicin C disaccharide, quanolirone disaccharide, lankocyclcine oligosaccharides, landomycin Y, etc. Synthesis of one representative example, the sugar part...