a-Hydroxy carboxylic acids and their derivatives are naturally occurring starting materials for the synthesis of functionalized tetrahydropyrans using ring closing metathesis and base induced epoxide rearrangement as key steps. (S)-Lactic acid methyl ester has been used as the starting material for the preparation of enantiomerically pure 6-desoxy C-glycoside precursors.Tetrahydropyrans with oxo substituents play an important role in synthetic organic chemistry, as these substructures are found in many natural products with interesting biological properties, such as annonaceous acetogenins, 1 polyether ionophores, 2 and C-glycosides. [3][4][5] The latter class of compounds has found increasing interest over the past years, because many physiologically active natural products with C-glycosidic structures have been identified, but there is also a continuous interest in the use of non-natural C-glycosides as carbohydrate mimics. 6 Thus, organic chemists have developed a variety of syntheses for these compounds, mostly starting from naturally occurring carbohydrates. Probably the most useful de novo synthesis of glycals and carbohydrates is the hetero-DielsAlder reaction of aldehydes and siloxydienes. 7,8 Metal-catalyzed or -mediated ring closure reactions are less common in this field. Among this group of reactions the ring closing metathesis of dienes 9 turned out to be a very powerful tool over the last few years. Recent examples for the utilization of olefin metathesis for the construction of di-and tetrahydropyran frameworks include the synthesis of spirocyclic dihydropyrans 10 and the utilization of carbohydrates as starting materials for fused or spiroannellated dihydropyrans and -furans. 11In this communication we wish to disclose our preliminary results on the utilization of naturally occurring a-hydroxy carboxylic acid 12 derivatives as starting materials for functionalized di-and tetrahydropyrans (Scheme 1). 13The a-allyloxy ester 2a was obtained in enantiomerically pure form and very high yield from inexpensive (S)-methyl lactate following a literature procedure. 14 Allylation of 1a with sodium hydride and allyl bromide occurs with complete racemization to give rac-2a ([a] 25 D = 0°). Reduction of the ester functionality in 2a with DIBAL-H and addition of one equivalent of vinylmagnesium chloride in a one-pot procedure yields, with good diastereoselectivity (dr = 4:1), the allylic alcohol 3a. The relative configuration of 3a is opposite to the one observed by us for very similar systems, which have been prepared via a two-step procedure, i. e. with isolation of the aldehyde. 13 Thus, it is likely that the formation of 3a via the one-pot procedure involves the intermediate formation of a chelate complex A of aluminum and the a-allyloxy aldehyde ("Cram's cyclic model") (Scheme 2). 15 Scheme 2Ring closing metathesis of 3a gives dihydropyrans 4a as an inseparable mixture of diastereomers (cis/trans = 4:1). The cis-configuration of the major diastereoisomer was elucidated by comparison of the coupling constant...

An investigation into the factors influencing the diastereochemical outcome of the ring-closing metathesis based synthesis of dihydropyrans is presented in this paper. Divinyl carbinols derived from alpha-hydroxy carboxylic acid esters are elaborated to trienes with two diastereotopic vinyl moieties. Depending on the steric demand of the oxo substituent of the divinyl carbinol moiety (either unprotected OH, TBDMS, or benzyl ether) different diastereomers are preferrably formed upon ring-closing metathesis. An extension to diastereoselective double ring-closing metathesis in the formation of spirocycles has also been investigated.

The preparation of dihydropyran and dihydrofuran oxides and their rearrangement in the presence of lithium dialkylamides to functionalized 2,3‐dihydropyrans or 2,3‐dihydrofurans, respectively, is described. The regiochemical outcome of the reaction can be influenced by the relative configuration of the starting epoxides and the steric demand of the base. The 2,3‐dihydropyrans obtained were converted stereoselectively to difunctionalized 3,4‐dihydropyrans by the carbon‐Ferrier reaction, or to fused acetals by addition of dimedone, mediated by ceric ammonium nitrate. The stereochemical results are rationalized by mechanistic proposals.

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reactions of organo-metal compounds reactions of organo-metal compounds O 0350 -065Synthesis of Enantiomerically Pure Divinyl-and Diallylcarbinols.-The novel synthesis of enantiomerically pure α-chiral divinyl-and diallylcarbinols involves the treatment of acylated bornane sultams and oxazolidinones with excess of vinyl-or allylmetal compounds. For the preparation of divinylcarbinols, acylated bornane sultams are the starting materials of choice, whereas diallylcarbinols can be prepared from both types of acyl derivatives. The chemoselective formation of derivative (XII) is in particular remarkable since the ester group is not attacked by application of stoichiometric amounts of vinyl metal compound (II). Two examples for the application of the obtained carbinols in metathesis reactions are given, as well as reaction conditions which lead to selective formation of chiral butenones such as (X).