“…Thus, the desired product 18 was obtained in 75% ee and 89% yield from 8d . This compound 18 can be easily converted into (+)-podocarpa-8(14)-en-13-one ( 19 ), 30a, a versatile intermediate for synthesis of naturally occurring diterpenes, e.g. isophyllocladene,35a phyllocladene,35a hibaone,35b manool,35c sclareol,35c manoyl oxide,35c isoabienol,35d trans -abienol,35d and anticopalic acid. 35e,f Therefore, the present work can be regarded as the enantioselective total synthesis of the above natural diterpenes.…”
An enantio- and diastereoselective stepwise cyclization of polyprenoids induced by Lewis acid-assisted chiral Brønsted acids (chiral LBAs) and achiral LBAs is described. In particular, the absolute stereocontrol in the initial cyclization of polyprenoids to form an A-ring induced by chiral LBAs and the importance of the nucleophilicity of the internal terminator in polyprenoids for the relative stereocontrol in subsequent cyclization are demonstrated. (-)-Ambrox was synthesized via the enantioselective cyclization of (E,E)-homofarnesyl triethylsilyl ether with tin(IV) chloride-coordinated (R)-2-(o-fluorobenzyloxy)-2'-hydroxy-1,1'-binaphthyl ((R)-BINOL-o-FBn) and subsequent diastereoselective cyclization with CF(3)CO(2)H.SnCl(4) as key steps. Protection of (E,E)-homofarnesol by a triethylsilyl group increased the enantioselectivity of chiral LBA-induced cyclization and both the chemical yield and diastereoselectivity in the subsequent cyclization. The enantioselective cyclization of homo(polyprenyl)arenes possessing an aryl group was also induced by (R)-BINOL-o-FBn.SnCl(4). Several optically active podocarpa-8,11,13-triene diterpenoids and (-)-tetracyclic polyprenoid of sedimentary origin were synthesized (75-80% ee) by the enantioselective cyclization of homo(polyprenyl)benzene derivatives induced by (R)-BINOL-o-FBn.SnCl(4) and subsequent diastereoselective cyclization induced by BF(3).Et(2)O/EtNO(2) or CF(3)CO(2)H .SnCl(4).
“…Thus, the desired product 18 was obtained in 75% ee and 89% yield from 8d . This compound 18 can be easily converted into (+)-podocarpa-8(14)-en-13-one ( 19 ), 30a, a versatile intermediate for synthesis of naturally occurring diterpenes, e.g. isophyllocladene,35a phyllocladene,35a hibaone,35b manool,35c sclareol,35c manoyl oxide,35c isoabienol,35d trans -abienol,35d and anticopalic acid. 35e,f Therefore, the present work can be regarded as the enantioselective total synthesis of the above natural diterpenes.…”
An enantio- and diastereoselective stepwise cyclization of polyprenoids induced by Lewis acid-assisted chiral Brønsted acids (chiral LBAs) and achiral LBAs is described. In particular, the absolute stereocontrol in the initial cyclization of polyprenoids to form an A-ring induced by chiral LBAs and the importance of the nucleophilicity of the internal terminator in polyprenoids for the relative stereocontrol in subsequent cyclization are demonstrated. (-)-Ambrox was synthesized via the enantioselective cyclization of (E,E)-homofarnesyl triethylsilyl ether with tin(IV) chloride-coordinated (R)-2-(o-fluorobenzyloxy)-2'-hydroxy-1,1'-binaphthyl ((R)-BINOL-o-FBn) and subsequent diastereoselective cyclization with CF(3)CO(2)H.SnCl(4) as key steps. Protection of (E,E)-homofarnesol by a triethylsilyl group increased the enantioselectivity of chiral LBA-induced cyclization and both the chemical yield and diastereoselectivity in the subsequent cyclization. The enantioselective cyclization of homo(polyprenyl)arenes possessing an aryl group was also induced by (R)-BINOL-o-FBn.SnCl(4). Several optically active podocarpa-8,11,13-triene diterpenoids and (-)-tetracyclic polyprenoid of sedimentary origin were synthesized (75-80% ee) by the enantioselective cyclization of homo(polyprenyl)benzene derivatives induced by (R)-BINOL-o-FBn.SnCl(4) and subsequent diastereoselective cyclization induced by BF(3).Et(2)O/EtNO(2) or CF(3)CO(2)H .SnCl(4).
“…This compound 8 can be easily converted into (+)-podocarpa-8(14)-en-13-one ( 9 ), 3a, a versatile intermediate for synthesis of naturally occurring diterpenes, e.g. isophyllocladene,9a phyllocladene,9a hibaone,9b manool,9c sclareol,9c manoyl oxide,9c isoabienol,9d trans -abienol,9d and anticopalic acid. 9e,f Therefore, the present work can be regarded as the enantioselective total syntheses of the above natural diterpenes. 2 Enantioselective Synthesis of (5 S ,10 S )- 8 …”
“…The α,β-enone 22 has been synthesized in our laboratory from tricyclic α,β-enone 30 , , via the isopropylidene ketal 32 , derived from abietane 31 . Compound 32 has also been easily prepared from abietic acid ( 26 ) (Scheme ).…”
A new strategy for synthesizing taiwaniaquinoids, a group of terpenoids with an unusual rearranged 5(6→7) or 6-nor-5(6→7)abeo-abietane skeleton, which exhibit promising biological activities, is reported. The procedure, based on the cleavage of the C7-C8 double bond of abietane diterpenes, is the only one yet reported for synthesizing C(20) taiwaniaquinoids bearing a carbon function on the cyclopentane B ring; it is also applicable to the synthesis of the wide variety of existing taiwaniaquinoids. Utilizing this, (-)-taiwaniaquinone A, F, G, and H, (-)-taiwaniaquinol B, and (-)-dichroanone have been synthesized from (+)-abietic acid. The versatility of this strategy allows us to propose the abietane C7-C8 cleavage as a possible biosynthetic pathway to this type of rearranged diterpenes; this proposal seems to be supported by phytochemical evidence.
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