
%0 Journal Article
%A Aggarwal, Rachna 
%A Birkbeck, Anthony A. 
%A Giles, Robin G. F. 
%A Green, Ivan R. 
%A Gruchlik, Yolanta 
%A Oosthuizen, Francois J. 
%T Regioselectivity in the Syntheses of Enantiopure 2-Benzopyrans through Intramolecular Cyclization of Tethered Lactaldehydes. Conformations of the Products
%J Australian Journal of Chemistry
%D 2003/05/23
%V 56
%N 5
%P 489-498
%K 
%R https://doi.org/10.1071/CH03027
%U https://www.publish.csiro.au/paper/CH03027 
%X Using titanium tetraisopropoxide, the enantiopure tethered lactaldehyde (&#945;&#8242; S,2S)-2-(3&#8242;-hydroxy-&#945;&#8242;-methyl-benzyloxy)propanal (6) is cyclized with complete regio- and diastereoselectivity ortho to the phenolic hydroxyl group to give (1S,3S,4R)-3,4-dihydro-1,3-dimethyl-2-benzopyran-4,5-diol (7). Similar cyclization of the epimeric (&#945;&#8242; R,2S)-lactaldehyde (25) yields solely the corresponding (1R,3S,4R)-4,5-diol (34). The 4,5-diacetate (26), but not (35), undergoes conformational inversion of the heterocyclic ring through significant 4,5-peri interactions between the adjacent acetoxy substituents. Spontaneous cyclizations of the corresponding phenoxide ions of the lactalde-hydes (6) and (25), generated by fluoride from their silyl ethers, led to the related 4,7-diols with high regioselectivity through ring-closure para to the aromatic oxygen. 
