“…362 An additional synthesis of (±)-capnell-9(12)-ene 511, a perennial favourite synthetic target from C. imbricata, 363 utilizes p-cresol as the starting material. 364 Five primnatriene sesquiterpenes 512-516 from the soft coral Primnoeides sp. 365 have been synthesized in a concise manner.…”
“…362 An additional synthesis of (±)-capnell-9(12)-ene 511, a perennial favourite synthetic target from C. imbricata, 363 utilizes p-cresol as the starting material. 364 Five primnatriene sesquiterpenes 512-516 from the soft coral Primnoeides sp. 365 have been synthesized in a concise manner.…”
“…Singh and co-workers 163 have developed a photochemical oxa-di-π-methane rearrangement-based route to (()-capnellene (282) (Scheme 79). The requisite bridged enone 320 was obtained via an inverseelectron demand Diels-Alder reaction between the dienone derived from the oxidation of 2,5-dimethylbenzyl alcohol and cyclopentadiene.…”
Section: B Capnellanesmentioning
confidence: 99%
“…In addition, diand sesterterpene natural products, like laurenene and retigeranic acid also incorporate an angular triquinane unit corresponding to silphinane and silphiperfolane, respectively. The main challenge in the synthesis of angular triquinanes is the installation of a network of methyl groups and quaternary Scheme 77 160,161 Scheme 78 162 Scheme 80 164,165 Scheme 76 159 Scheme 79 163 Synthesis carbon centers in addition to the control of stereochemistry of the remote secondary methyl group.…”
“…Although not extensively, the Diels−Alder chemistry of cyclohexadienones I − VII (Figure ) has been studied and used in the total synthesis of natural products. ,,− The use of cyclohexadienones I − IV in a general way is constrained in that their preparation requires strategic placement of substituents, which cannot be removed easily. On the other hand, 2,4-cyclohexadienones of type V − VII , which may also be called as o -quinone monoacetals or masked o -benzoquinones (MOBs), could be ideal compounds for use in Diels−Alder reactions because the unavoidable acetal group, being positioned next to a keto group, could be removed by reduction if not required or modified into a desired functionality at a latter stage with relative ease. , However, lack of efficient methods for their preparation and their high propensity to dimerize are main deterrents for their use in organic synthesis. , 1 Various types of 2,4-cyclohexadienones.…”
Diels−Alder reactions of labile and readily dimerizing masked o-benzoquinones, i.e., substituted
6,6-dimethoxy-2,4-cyclohexadienones with electron-deficient dienophiles that resulted in the
development of an efficient and reliable one-pot method for the preparation of highly functionalized
bicyclo[2.2.2]octenones, are described. Oxidation of 2-methoxyphenols 8, 9, 11, 13, and 14 with
(diacetoxy)iodobenzene in methanol afforded the corresponding masked o-benzoquinones 1, 2, 4,
6, and 7, which are not stable enough to be isolated and are found to dimerize under reaction
conditions in a highly regio- and stereoselective manner to provide the Diels−Alder dimers 22, 23,
25, 27, and 28 respectively. On the other hand, masked o-benzoquinones 3 and 5, derived from
phenols 10 and 12, respectively, were found to be quite labile and provided a complex mixture of
products. However, masked o-benzoquinones 1−7, when generated in the presence of dienophiles
such as methyl acrylate, methyl methacrylate, and methyl vinyl ketone, underwent highly regio-
and stereoselective intermolecular Diels−Alder reactions to furnish variously substituted bicyclo[2.2.2]octenones 15−21(a−c). While the Diels−Alder reactions of masked o-benzoquinone 5 were
found to be low-yielding, masked o-benzoquinones 1−4, 6, and 7 provided the desired adducts in
good to high yields. Attempts are made to explain the observed regio- and stereoselectivity of these
Diels−Alder reactions in terms of frontier molecular orbital theory.
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