Bradford Scholars -how to deposit your paper Overview Copyright check• Check if your publisher allows submission to a repository.• Use the Sherpa RoMEO database if you are not sure about your publisher's position or email openaccess@bradford.ac.uk.

[reaction: see text] Diels-Alder reactions of 3- and 5-halo-subsituted 2(H)-pyran-2-ones with both electron-rich and electron-deficient dienophiles afford stable and readily isolable bridged bicyclic lactone cycloadducts. These cycloadditions proceed with excellent regioselectivity and very good stereoselectivity. In contrast, Diels-Alder reactions of 4-halo-subsituted 2(H)-pyran-2-ones afford cycloadducts which are very prone to loss of bridging CO(2) and the subsequent formation of barrelenes ([2.2.2]cyclooctenes). Furthermore, these cycloadditions proceed with only moderate regio- and stereoselectivity. For both series of the 3- and 5-halo-subsituted 2(H)-pyran-2-ones and 4-halo-subsituted 2(H)-pyran-2-ones, the reactivity patterns do not significantly change between the halogens. The regio- and stereochemical preferences of the cycloadditions of halo-substituted 2(H)-pyran-2-ones are investigated computationally. Calculations were carried out on the transition states leading to the four possible regio- and stereoisomeric cycloadducts by using density functional theory (B3LYP/6-31G). These studies allow prediction of the regio- and stereoselectivity in these reactions which are broadly in line with experimental observations.

No abstract

Transannular Diels–Alder reactions on 14-membered macrocycles containing properly located diene and methyl-substituted dienophile units lead to A.B.C.[6.6.6] tricycles related to steroids. To study the influence of the olefin and diene geometry on the stereochemical outcome of the Diels–Alder reaction it was necessary to prepare macrocyclic trienes of well-defined stereochemistry. Eight different types of macrocyclic trienes might be obtained by the coupling of appropriate dienophiles and dienes, namely, TTC, TTT, TCC, TCT, CTC, CTT, CCC, and CCT. In this paper (first in a series of two), the synthesis of appropriately functionalized dienophile and diene synthons, as well as their coupling reaction affording macrocyclic triene precursors, is described.