DFT calculations show that square‐planar LCoIR complexes of a diiminopyridine ligand are best regarded as containing low‐spin CoII antiferromagnetically coupled to a ligand radical anion. The lowest triplet state, corresponding to a 3d italicz 2⇄π* excitation, is calculated to be only a few kcal/mol above the ground state, and is thermally accessible. The anomalous 1H NMR chemical shifts of the LCoR complexes are suggested to be due to thermal population of the triplet state at room temperature. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

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A straightforward route for the synthesis of α,α'-disubstituted dihydropyrans and tetrahydrooxepines has been developed involving Pd-and Ru-catalyzed reactions.2,6-Disubstituted dihydropyrans 1 and 2 (n = 1) occur frequently in nature as a structural unit in a wide range of natural products. 1 In addition, these compounds could serve as useful building blocks for the synthesis of biologically active saturated oxygen heterocycles via functionalization of the double bond. Examples include (poly)cyclic ethers, 2 but also modified carbohydrates and 2-carboxyl-substituted tetrahydropyrans with antibacterial activity such as KDO. 3 More specifically, dihydropyrans 2 (R 1 = OMe) have recently been used as intermediates in the synthesis of mevinolin and compactin. 4,5 In conjunction with previous work in our group on vinylsilane-terminated formation of dihydropyrans via oxycarbenium ions, 6 we set out to explore a novel entry into this compound class, which would enable us to vary the side chains and would also give facile access to enantiopure heterocycles. Moreover, the methodology outlined in this paper is not restricted to six-membered ring formation, but can also be applied to form the corresponding seven-membered rings. Especially the latter aspect provides a significant benefit compared to previously established methods for forming such rings via hetero Diels-Alder reactions. 5,7 A short retrosynthetic outline of our route is shown in Scheme 1. Scheme 1The target compounds 1 and 2 should be accessible via the allylic acetal 3, which contains (i) a terminal olefin function that can be used as a handle for ring-closing metathesis 8,9 and (ii) an acetal function to enable later modification of the system via oxycarbenium ion chemistry.The first sequence (Scheme 2) commenced with anhydrous methyl glyoxylate (5), 10 which upon treatment with BF 3 ·OEt 2 and allyltrimethylsilane led to methyl 2-hydroxy-4-pentenoate (6). Initially, attempts were made to convert 6 into the desired allylic acetal under acid-catalyzed conditions with acrolein derivatives, but without much success. Therefore, we turned our attention to other strategies, involving the use of 1-methoxy-1,2-propadiene, 11 which upon electrophilic activation and subsequent reaction with the secondary alcohol might give rise to the desired product. Several methods have been published describing activation of allenes towards coupling with oxygen nucleophiles (including N-bromosuccinimide, HgCl 2 ), 11 but these routes are not attractive in view of the stoichiometric use of the reagents. At this point, a publication of Alper and coworkers appeared, 12 who obtained a similar acetal as a byproduct in Pd(II)-catalyzed annulation reactions with substituted allenes. Application of their conditions and further optimization (Pd(OAc) 2 (cat.), Ph 2 P(CH 2 ) 3 PPh 2 (dppp), excess of 1-methoxy-1,2-propadiene, Et 3 N, MeCN, 80 °C in a sealed tube) led to a virtually quantitative yield of the desired acetal 7. 13 Not surprisingly, 7 was obtained as a ca. 1:1 mixture of diaste...