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Relevant academic research and scientific papers

Stereochemistry of addition of organometallic reagents to 2-acyloxanes and 2-acylthianes

The stereochemistry of addition of a number of organometallic reagents, including methyllithium, methyllithium-YbCl3, 1-pentynyllithium, and 1-pentynyllithium-YbCl3 to 2-acyloxanes, 2-benzoylthianes, several substituted 1,3-oxathianes and 2-methoxyacetylthiane has been studied. For the ring systems containing oxygen atoms, almost all of the additions proceed in accordance with Cram's chelate rule, on the assumption that the chelation takes place with the oxygen rather than the sulfur atoms of the ring where both are present. An exception (cf. Utimoto et al.7) is the addition of 1-pentynyllithium-YbCl3 to conformationally locked 1,3-oxathianes; however, addition of methyllithium-YbCl3 to all 1,3-oxathianes studied and of pentynyllithium-YbCl3 to the conformationally mobile 2-benzoyl-1,3-oxathiane proceeds 'normally'. Additions to 2-benzoylthiane are usually of low stereoselectivity and often proceed contrary to what would be predicted on the assumption that the organometallic reagent chelates with sulfur; an exception is methylmagnesium iodide which does follow Cram's chelate rule. The additions of RLi and RLi.YbCl3 (R methyl or 1-pentynyl) to 2-benzoylthiane follow the same stereochemical course presumably proceeding contrary to Cram's chelate rule; thus the reversal observed with 1-pentynyllithium-YbCl3 in the 1,3-oxathiane series appears not to be due to chelation with sulfur. The results with 2-methoxyacetylthiane suggest that chelation with the side-chain oxygen substituent prevails over chelation with the ring sulfur atom.

Chemistry of 1,3-Oxathianes. Synthesis and Conformation of 2-Substituted 1,3-Oxathianes

The reaction of 2-lithio-1,3-oxathiane with a variety of electrophiles including alkyl halides and carbonyl compounds affords substitution and addition products, respectively.Alkyl metal halides such as (CH3)3SiCl, (CH3)3GeCl, and (CH3)3SnCl, or alkyl metal acetate, (CH3)3PbOAc, reacted with 1,3-oxathianyl anion to give rise to the corresponding 2-(group 14)-substituted 1,3-oxathianes (4). 2-(Methylthio)- and 2-(methylseleno)-1,3-oxathianes (6b and 6c) were prepared by a similar reaction of the anion with dimethyl disulfide and dimethyl diselenide, respectively.Other1,3-oxathianes substituted with a 2-methoxy or 2-dimethylamino group at C-2 have been synthesized to investigate the orientation of heteroatoms.Examination of their 1H and 13C NMR spectra leads to the conclusion that group 14 metals and a dimethylamino group occupy the equatorial position, while methoxy and methylthio groups favor the axial disposition due to the anomeric effect.Conclusive evidence was not obtained from the available data to decide whether 2-(methylseleno)-1,3-oxathiane (6c) exhibits the anomeric effect or not.

SYNTHESIS OF 2-HETEROSUBSTITUTED 1,3-OXATHIANES AND THEIR REACTION WITH sec-BUTILLITHIUM

A series of 2-heterosubstituted 1,3-oxathianes were synthesized and their reactivity toward a strong base, sec-butyllithium, was studied.It was shown that sec-butyllithium attacked 2-heterosubstituted 1,3-oxathianes in three different ways depending upon the heteroatom at C-2; the removal of the proton at C-2, the nucleophilic attack at heteroatom, or the nucleophilic attack at C-2 took place.