Synthesis of o-isopentenylphenols

Abstract

The effect of the solvent and the counter-ion on the ring alkylation of 3- and 4-methoxyphenols by isopentenyl bromide has been examined. Under strictly anhydrous conditions good and consistent yields of the o- isopentenylphenols were obtained with the combination potassium- toluene, but sodium-toluene was more convenient. ��� Boron trifluoride etherate was more satisfactory than stannic chloride, zinc chloride, or phosphoric acid in catalysing the condensation of 3- and 4-methoxy-phenols with 2-methylbut-3-en-2-ol to the corresponding o-isopentenylphenols. Aryl α,α-dimethylpropargyl ethers were partially hydrogenated to the corresponding α,α- dimethylallyl ethers which rearranged in boiling N,N-diethyl-aniline to o-isopentenylphenols in high overall yield.

Unified synthesis of caged Garcinia natural products based on a site-selective Claisen/Diels–Alder/Claisen rearrangement

“…Several conditions were tested for the conversion of 18 to the tris(dimethylallyloxy)xanthone 17a (22). The most efficient proved to be a two-step procedure that involved propargylation of the more reactive C3, C5, and C6 phenols with 2-chloro-2-methylbutyne (21) in the presence of KI and K 2 CO 3 under CuI catalysis (25% yield) (23), followed by Lindlar reduction of the pendant alkynes (75% yield) (24). Drawing on previous experience with related structures (25,26), we heated 17a in DMF at 120°C and obtained a mixture of forbesione (1a) (49% yield) and isoforbesione (22a) (35% yield) (18).…”

The Claisen rearrangement of 1,4-bis(m-methoxyphenoxy-)2-butyne

“…Though unstable, such structures have actually been isolated (2) in the Claisen rearrangement of several aryl propargyl ethers to chromenes. The presence of large quantities of chromenes (2) shortly after the start of the reaction indicates that the second step involves a rapid [1,5] hydrogen shift, to yield the w-vinylquinone methide 9, and subsequent electrocyclic ring closure. As an alternate possibility, 8 may undergo another Claisen rearrangement to a 2,3-diarylbutadiene (10) from which 3, 4, and the minor by-prod;ct 11 could be formed directly by two successive cyclizations (16).…”

“…Chromene 2 is converted to methylpterocarpan 3 (and traces of 11) via a 3-aryl-4-exomethylenebenzopyran (12) by a second [3,3] sigmatropic shift (normal Claisen rearrangement) and a homodienyl [1,5] sigmatropic shift (abnormal Claisen rearrangement) (lo), and the latter undergoes irreversible ring ccntractlon to a benzofurobenzofuran (4 and (or) 5). The rate acceleration observed on addition of Lewis acids suggests the formation of a common resonance-stabilized benzylic carbocation intermediate (13) by acid-catalyzed cleavage of the pyran ring of 3 (4).…”

“…The Claisen rearrangement of aryl propargyl ethers is a convenient route for the preparation of chromenes (1,2). With 1,4-bis(ary1oxy-)2-butynes it yields 6H-benzofuro [3,2- (3)(4)(5) in the formation of 1 la-methylpterocarpans, and the high thermodynamic stability of the cyclic ketals 5, which are generally isolated as end products after long reaction times at high temperatures (4)(5)(6)(7)(8), especially in the presence of acid catalysts (Scheme 1).…”

Synthesis of the phytoalexin (±)-phaseollin: 3-phenylthiochromans as masked 2H-chromenes and o-prenyl phenols

“…Similar reactions were carried out with the phenylthiochromans (lo), (13), and (14), and reductive ring opening proceeded satisfactorily in each case, without carbonyl reduction or interference by free hydroxy functions. In this way the phenol (28) l 7 was obtained [59% and 49% from methods (ii) and (iii)] from the chroman (lo), and also from the corresponding sulphone (12) [method (ii); 25x1. Reduction of the sulphide (13) gave the geranylphenol(29) (49%), as a 1 : 1 mixture of E and Z Finally, the trihydric geranylacetophenone (30) was obtained, as an unstable oil.…”