Olefin Metathesis Directed to Organic Synthesis: Principles and Applications

Synthesis of Dihydrofurans and Dihydropyrans with Unsaturated Side Chains Based on Ring Size‐Selective Ring‐Closing Metathesis

“…Surprisingly, a small amount of monocyclized products 7a and 8a could be detected with phosphine-free catalysts D and E (entries 3 and 4) while A and C (normally considered to be less active) lead to the exclusive formation of bicyclic products 9 and 10. With first generation catalyst A the reaction is highly selective, giving exclusively the bis-dihydrofuran 10 ( 1 H NMR, entry 1), whereas C results in the formation of fused (9) and dumbbell-type product (10) in a ratio of 1.0:1.7 (entry 2). Both bicyclic products could be isolated and were characterized by one-and twodimensional NMR spectroscopy.…”

“…[9,10] Figure 2 shows the structures of the first-generation ruthenium precatalysts A [41] and B [42] and the second-generation precatalysts C, [43] D and E [44] which have been used in the course of this study.…”

“…[7] It is an interesting feature of the structural motifs I-III that they display a common skeleton of carbon and oxygen atoms, which is highlighted in Scheme 1. In continuing previous works in our group dedicated to the exploitation of the olefin metathesis reaction [8][9][10][11][12][13] for the synthesis of functionalized oxacycles, [14][15][16][17][18][19] we wondered if the common arrangement of carbon and oxygen atoms in I, II and III could be accessed by a common precursor, hexadienediol 1, after its transformation to tri-or tetraenes 2 (Scheme 1). This would be particularly attractive from the synthetic point of view, as 1 is easily available in multi-gram quantities from d-mannitol, [20,21] while its enantiomer ent-1 can be synthesized from R,R-dimethyl tartrate.…”

Palladium‐Catalyzed O‐Allylation of α‐Hydroxy Carbonyl Compounds

“…[22,23] We have successfully used the latter method for a-hydroxy esters. [2,3,5] However, application of these conditions to aromatic a-hydroxy ketones [24] turned out to give less satisfactory results, because these substrates partially undergo an oxidative C À C bond cleavage under these conditions, resulting in the formation of allyl benzoate (8) as a by-product. For 3-hydroxypropiophenone (6) and substituted derivatives this side reaction becomes a serious limitation, as approximately 50% of the starting material are converted via this pathway (Scheme 2).…”

“…[2 -7] Key features of our strategy are i) a selective O-allylation of the corresponding a-hydroxy carbonyl compound 1, ii) the diastereoselective addition of vinyl-or allylmetal compounds to the resulting O-allyl ethers 2, iii) the Ru-catalyzed ring closing metathesis of dienes 3, [8,9] which yields dihydropyrans (n ¼ 0) or oxepins (n ¼ 1) 4, respectively (Scheme 1). Oxacycles 4 have great potential in the de novo synthesis of carbohydrates or related compounds, [10,11] which has been demonstrated by us [5,12 -14] and others.…”