2- -4//-1 -benzopyran-4-one

Benzopyran-3-one, l,4-dihydro-6,7-dimethoxy-], 55, 45 Isocyamde, benzyl-, 55, 98 Isocyanide, butyl-, 55, 98 ISOCYANIDE, tert-butyl- [Isocyamde, 1,1-dimethylethyl-], 55, 96 Isocyamde, cyclohexyl-, 55, 98 Isocyamde, dodecanyl-, 55, 98 Isocyamde, ethyl, 55,98 Isocyamde, methyl, 55, 98 Isocyamde, phenyl-, 55, 98 (-)-2,3 4,6-Di-0-isopropylidene-2-keto-L-gulomc acid, hydrate [L-jcy/o-2-Hex-ulosomc acid, bis-<9-( 1 -mcthylcthyl-ldene)-], 55,80, 81 ISOXAZOl F, 3-(4-chlorophenv1)-5-(4-methoxyphenyl)-, 55, 39 Isoxazole,5 -(4-chloropheny l)-3-(4-me th-oxyphenyl)-, 55,42... 

Benzoic acid, 1,1-dimethylethyl ester [Benzoic acid, fe/7-butyl ester] as impurity in /erV-butyllithium, 55, 125 Benzoic acid, 4 methoxy, methyl ester [Anisic acid, methyl ester], 55, 40, 41 3//-2-BENZOPYRAN-3-ONE, 1,4-dihydro-... 

Both inter- and intra- molecular Diels-Alder reactions of 2-benzopyran-3-ones occur with high endo-selectivity and have been used to synthesise (-)-podophyllotoxin (14) and 4a-substituted cis-BC fused hexahydrophenenthrenes (15), respectively <96JCS(P1)151, 96JCS(P1)705>. 

Complete aromaticity in coumarin (19), isocoumarin (28) and 3H- 2- benzopyran-3-one (206) can only be realized if the O—CO function contributes two electrons to form a 1077-electron system, which would necessarily possess a betaine structure. 

The Reformatsky reaction has been applied to the synthesis of fused pyranones and provides an example of selective isomer formation by careful choice of substrate. Hydroxy-methylenecyclohexanone (315) and methyl bromoacetate give 5,6,7,8-tetrahydro-l-benzopyran-2-one (316) through alkylation at the hydroxymethylene carbon atom (54JA6388). However, the benzoyl derivative (317) is unable to form an enolate salt and alkylation occurs at the carbonyl carbon atom, leading to 5,6,7,8-tetrahydro-2-benzopyran-3-one (318) (45HCA771). 

The use of isopropylidine acetals (112) as tethers in the intramolecular Diels-Alder reactions of dienes with alkenes facilitates the formation of civ-fused cycloadducts (113) from an endo transition state (Scheme 41).218 The intramolecular Diels-Alder reaction of 4-[tris-(2-mcthylcthyl)silyl]oxy-2//-thiopyran derivatives with potential dienophiles tethered at C(2), C(3), C(5), and C(6) positions yielded cycloadducts when the dienophiles were activated with a carbomethoxy group.219 By the substitution of a phenylsulfonyl group on the dienophile of 2-benzopyran-3-ones, it is possible to enhance exo addition during intramolecular Diels-Alder cyclizations to yield a predominance of trans-fused hexaphenanthrenes related to natural products.220 The intramolecular Diels-Alder reaction of 2-furfuryl fumarates has been investigated by molecular mechanics (SIBFA)/continuum reaction field computations.221 The intramolecular 4 + 2-photo-cycloaddition of A-benzylcinnamamides (114) in the presence of C(,H6 gives 3-azatricyclo[5.2.2.01,5]undeca-8,10-dien-4-ones (115) with high stereoselectivity (Scheme 42).222... 

Benzopyran-3-ones are not generally isolated, but they are useful as reactive intermediates and especially as reactive Diels-Alder dienes. The generation of 3/7-2-benzopyran-3-ones is usually achieved by the cyclodehydration of ortfo-acylphenylacetic acids carried out in the presence of acetic anhydride (Scheme 222) <1996CHEC-II>. 

Numerous examples of exo selective cycloadditions employing a diversity of diene partners attest to the generality of this manner of combination. Isodicyclopentadiene can be added to tropone to give principally ketone (4) and in situ generated 2-benzopyran-3-one provided o-lactone (5) in 28% yield. 1-Substituted butadienes have proven to be particularly useful 4 parmers, which give the exo adducts (6) exclusively when reacted with tropone at moderate temperatures. Furthermore, the intramolecular vari-... 

Benzopyran-3-one 399 is also a very reactive diene in the intermolecu-lar Diels-Alder reaction [70JCS(C)536 76JCS(P1)1647]. (See Schemes 74 and 75.) 2-Benzopyran-3-one 399a-b derived from 400a-b undergoes... 

Benzopyran-3-ones, generated by cyclising dehydration of an ort/to-formyl-arylacetic acids take part in intramolecular Diels-Alder additions, as shown below. ... 

In contrast, the adduct (41) obtained from singlet oxygen and l,4-diphenyl-3/7-2-benzopyran-3-one (40) is a stable, crystalline solid (Scheme 2) <78JA2564>. Nonetheless, (41) decomposes in boiling benzene giving o-dibenzoylbenzene (43) and phenyl o-benzoylbenzoate (45) in 85% and 5% yield respectively. No luminescence is detected. The major reaction course is decarboxylation to the intermediate o-xylylene peroxide (42), which can be trapped as its Diels-Alder adduct with maleic anhydride. The ester (45) could have arisen directly from (41) by decarbonylation and concomitant... 

Benzodioxaborole (cateckolborane), 131 Benzoic add Birch reduction, 103-104 —, 2-(acetylnitrosoamino)- benzyne from, 93 Benzoin. See Ethanone, 2-hydroxy-l,2-diphenyl-Benzophenone = diphenyhnethanone aldol addn. and condensation with, 56-57 3tf-2 Benzopyran-3-one, 1,4-dihydro-, 80 Benzoquinones. See Cyclohexadienediones Iff-Benzotriazole, 1-hydroxy- (HBT, HOBT) acylation catalyst, 145, 231 Benzvalene — t ricyclo[2.1.1.03,6]hex-2-ene. 330-331... 

A new cathodic cyclodimerisation of 4-pyrones yields either (90) or (91) depending on conditions,88 2-Benzopyran-3-one has been isolated at low temperature and intercepted by NPM as the adduct (92) 88 high-pressure cycloaddition reactions of 2H-pyran-3-ones with a variety of dienophiles have been recorded.88 Cycloaddition reactions of furans and isobenzofurans continue to dominate this section. Dienophiles whose reactions with furan have been studied recently include phenylsulphonylpropadiene (giving predominantly the endo-adduct (93) 8b. benzynes (derived from aryloxazoli nesJ.8 and... 

The stereochemistry of the products of intramolecular Diels-Alder reactions of 4-substituted 2-benzopyran-3-ones is influenced by the nature of the terminal substituent X. Thus, (B-unsubstituted compounds yield the endo-chain adduct (42), whereas when X is -S02Ph exo-chain addition is predominant. Subsequent reductive elimination gives reduced phenanthracenes (Scheme 10) <97JCS(P1)1929>. 

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