Preparation and properties of some phytotoxic 2-benzyloxy-8-oxabicyclo[3.2.1]octane derivatives

Total synthesis of (±)-β-bulnesene via intramolecular cycloaddition of a 2-substituted 3-oxidopyrylium

“…Treatment of the tertiary alcohols (16) with thionyl chloride in hexamethylphosphoric triamide (HMPA) gave, in very high yield, the chlorides (17). The stereospecific nature of this substitution had previously been noted for the secondary alcohol (18), 6 which gave only the chloride (19), with retention of configuration. Although thionyl chloride is quoted as the classical reagent for S,i reactions, such processes rarely occur selectively.…”

“…Oxidation of the furfuryl alcohol (8) with rn-chloroperbenzoic acid afforded, as a mixture of epimers, the substituted 6hydroxypyranone (lo), which was readily acetylated with acetic anhydride in pyridine to produce the acetate (7). The latter when heated in acetonitrile at 150 "C for 20 h was converted into a mixture of the perhydroazulene cycloadducts (6) and (11) in 75% yield. Presumably the reaction proceeds by intermediate generation of the 3-oxidopyrylium ylide (12), which undergoes spontaneous intramolecular 1,3-dipolar cycloaddition across the olefinic bond.…”

“…First, the epimeric alcohols are known to undergo a smooth rearrangement to a bicyclo[4.4.0]decane under the influence of reagents such as thionyl chloride in HMPA' and second, in a biological test system, the benzyl ethers of the epimeric alcohols are active as herbicides whereas the benzyl ethers of the isomers, for example of (18), are inactive. 6 Cleavage of the epoxy bridge and introduction of the tetraalkyl substituted C( 1)-C(2) olefinic bond, characteristic of the guaianes (1)--(3), was achieved by reduction of the chloro ethers (17) with sodium in ether. The alcohols (21) were oxidised with chromium trioxide in pyridine to give the ketones (22) in 70% yield.…”

Oxidopyrylium-Alkene [5 + 2] Cycloaddition Conjugate Addition Cascade (C3) Sequences: Scope, Limitation, and Computational Investigations

“…N -methylpyrrolidine (cf. Table , entry 6) was added (0.4 mmol, 4.0 equiv) and the vial was backfilled with Ar, capped, and stirred for 6 h. Acetic acid (0.44 mmol, 4.4 equiv) was added to quench the reaction and the resulting solution was concentrated and the yield determined by NMR analysis with trimethoxybenzene as an internal standard and spectral data was consistent with previously published data: 1 H NMR (400 MHz, CDCl 3 ) δ 7.13 (dd, J = 9.8, 4.4 Hz, 1H), 5.96 (d, J = 9.8, 1H), 4.88 (dd, J = 6.6, 4.4 Hz, 1H), 2.45–2.38 (m, 1H), 2.35–2.28 (m, 1H), 2.16 (dd, J = 12.0, 8.8 Hz, 1H), 1.98–1.76 (m, 4H), 1.73–1.68 (m, 1H), 1.63–1.57 (m, 1H) …”

Synthesis of (±)-cryptofauronol and related valerane sesquiterpenes via rearrangement of bicyclo[5.3.0]decane precursors

“…Acetylation of 6-Hydroxypyran-2(6H)-ones (3) and (5).-6-Hydroxy-2-(4-methylpent-4-enyl)-2H-pyran-3(6H)-one (5) (39.3 g, 0.20 mol) was dissolved in dichloromethane (300 ml) containing pyridine (22.6 ml, 1.4 equiv. ), and acetyl chloride (17.1 ml, 1.2 equiv.)…”