Abstract
The iodine-catalyzed Prins cyclization of homoallylic alcohols and aldehydes was investigated under metal-free conditions and without additives. Anhydrous conditions and inert atmosphere are not required. The reaction of 2-(3,4-dihydronaphthalen-1-yl)propan-1-ol and 21 aldehydes (aliphatic and aromatic) in CH2Cl2 in the presence of 5 mol % of iodine gave 1,4,5,6-tetrahydro-2H-benzo[f]isochromenes in 54%–86% yield. Under similar conditions, the Prins cyclization of six alcohols containing an endocyclic double bond (primary, secondary, or tertiary) led to dihydropyrans in 52%–91% yield. The acyclic homoallylic alcohols gave 4-iodo-tetrahydropyran in 29%–41% yield in the presence of 50 mol % of iodine. This type of substrate is the main limitation of the methodology. The relative configuration of the products was assigned by NMR and X-ray analysis. The mechanism and the ratio of the products are discussed, based on DFT calculations.
Highlights
The Prins cyclization is a powerful method for the synthesis of hydropyrans [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]
Aiming to synthesize O-heterocyclic compounds, we decided to investigate the reaction of the homoallylic alcohol 1a with iodine in the presence of NaHCO3
Anhydrous conditions and inert atmosphere are not required in this metal-free protocol
Summary
The Prins cyclization is a powerful method for the synthesis of hydropyrans [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]. Several natural products were obtained using this reaction as an important step [7,8,20,21,22,23,24,25]. This transformation is carried out treating a mixture of a homoallylic alcohol and a carbonyl compound in the presence of an acid (Bronsted or a Lewis) (Scheme 1). One of the possible Lewis acids for Prins cyclization is iodine [26,27,28], which was used in stoichiometric amount in the presence of excess of homoallylic alcohols [29,30], we describe that a series of new pyrans can be obtained through.
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