An Enantiospecific Approach
to the 5-8-5 Tricyclic System of Basmanes

A. Srikrishna; Gopalasetty Nagaraju; G. Ravi

doi:10.1055/s-0030-1259073

LETTER

An Enantiospecific Approach to the 5-8-5 Tricyclic System of Basmanes

A. Srikrishna*, Gopalasetty Nagaraju, G. Ravi
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India
Fax: +91(80)3600683; e-Mail: askiisc@gmail.com;

Abstract

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Abstract

An enantiospecific synthesis of the 5-8-5 tricyclic ring system present in the basmane diterpenes has been accomplished, starting from ethyl 3-isopropyl-2-methylene-1-methylcyclopentaneacetate [readily available in five steps from (R)-limonene] employing an RCM reaction for the annulation of cyclooctane and an intramolecular rhodium carbenoid CH insertion reaction for the construction of the cyclopentane ring.

Key words

basmanes - fusicoccanes - enantiospecific synthesis - metathesis - rhodium carbenoid CH insertion

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References and Notes

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In addition varying amounts (10-20%) of another easily separable isomer of the diol 13, perhaps epimeric at the tertiary alcohol, was also obtained.

Yields refer to isolated and chromatographically pure compounds. All the compounds exhibited analytical data (IR, ^¹H and ^¹³C NMR, and HRMS) consistent with their structures.
Selected Data
1-[(1 S ,2 R ,3 S )-2-Allyl-2-hydroxy-3-isopropyl-1-methylcyclopentyl]pent-4-en-2-one (14) [α]_D ^²³ -18.4 (c 2.2, CHCl₃). IR (neat): ν_max = 3475 (OH), 3076, 2953, 2871, 1702 (C=O), 1458, 1380, 1060, 1002, 993, 913 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 6.05-5.75 (2 H, m, 2 × CH=CH₂), 5.19 (1 H, d, J = 10.2 Hz), 5.14 (1 H, d, J = 17.8 Hz), 5.06 (1 H, d, J = 9.6 Hz), 4.99 (1 H, d, J = 17.1 Hz), 3.37 (1 H, s, OH), 3.15 (2 H, d, J = 7.0 Hz, H-3), 2.66 (1 H, d, J = 18.3 Hz, H-1A), 2.58 (1 H, d, J = 18.3 Hz, H-1B), 2.20-1.80 (2 H, m), 1.88-1.65 (2 H, m), 1.62-1.46 (2 H, m), 1.42-1.30 (2 H, m), 0.99 (3 H, s, tert CH₃), 0.94 and 0.89 (6 H, 2 × d, J = 6.5 Hz, CH ₃CHCH ₃). ^¹³C NMR (100 MHz, CDCl₃): δ = 210.2 (C, C=O), 135.4 and 130.2 (2 × CH, CH=CH₂), 119.1 and 117.2 (2 × CH₂, CH=CH₂), 81.4 (C), 51.7 (CH), 50.0 (CH₂), 48.7 (CH₂), 48.1 (C), 46.0 (CH₂), 37.4 (CH₂), 29.1 (CH), 25.0 (CH₂), 23.9 (CH₃), 20.8 (CH₃), 20.3 (CH₃). HRMS: m/z calcd for C₁₇H₂₈O₂Na [M + Na]: 287.1987; found: 287.1984.

(1 S ,8 R ,9 S )-8-Hydroxy-9-isopropyl-1-methylbicyclo[6.3.0]undec-5-en-3-one (16)
[α]_D ^²7 +36.6 (c 3.9, CHCl₃). IR (neat): ν_max = 3517 (OH), 3031, 2954, 2872, 1690 (C = O), 1463, 1381, 1365, 1299, 1175, 1129, 1076, 1023, 930, 756 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 5.86 (1 H, q, J = 9.1 Hz), 5.65 (1 H, dt, J = 11.7, 6.2 Hz, CH=CH), 3.02 (1 H, dd, J = 18.5, 5.2 Hz), 2.92 (1 H, dd, J = 18.5, 7.0 Hz), 2.80 (1 H, d, J = 12.0 Hz), 2.35 (1 H, dd, J = 14.2, 9.1 Hz), 2.12 (1 H, dd, J = 14.2, 8.2 Hz), 2.01 (1 H, d, J = 11.9 Hz), 1.92-1.26 (7 H, m), 1.01 (3 H, s, tert CH₃), 1.04 (3 H, d, J = 6.3 Hz), 0.92 (3 H, d, J = 6.2 Hz, CH ₃CHCH ₃). ^¹³C NMR (100 MHz, CDCl₃): δ = 210.4 (C, C=O), 130.4 (CH) and 125.0 (CH, CH=CH), 83.9 (C), 50.6 (C), 50.3 (CH), 50.1 (CH₂), 44.0 (CH₂), 39.6 (CH₂), 36.4 (CH₂), 28.8 (CH), 24.5 (CH₂), 23.0 (CH₃), 21.4 (CH₃), 20.4 (CH₃). HRMS: m/z calcd for C₁₅H₂₄O₂Na [M + Na]: 259.1674; found: 259.1679.
(1 R ,2 S ,5 S ,9 S )-2-Isopropyl-5-methyl-12-oxatricyclo[7.2.1.0 ^¹,5 ]dodecan-7-one (17) [α]_D ^²4 +176.2 (c 1.3, CHCl₃). IR (neat): ν_max = 2953, 2868, 1697 (C=O), 1472, 1379, 1363, 1322, 1225, 1163, 1145, 1082, 1010 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 4.51-4.44 (1 H, m), 2.93 (1 H, dd, J = 17.8, 2.5 Hz), 2.55-2.30 (4 H, m), 2.26-2.14 (1 H, m), 1.90-1.40 (6 H, m), 1.38-1.20 (2 H, m), 1.04 (3 H, s, tert CH₃), 1.00 (3 H, d, J = 6.1 Hz), 0.85 (3 H, d, J = 6.5 Hz, CH₃CHCH₃). ^¹³C NMR (100 MHz, CDCl₃): δ = 211.2 (C, C=O), 94.3 (C, C-1), 74.1 (CH, C-9), 55.7 (CH, C-2), 53.0 (CH₂), 52.8 (CH₂), 44.8 (C, C-5), 40.5 (CH₂), 36.3 (CH₂), 30.8 (CH, CHMe₂), 29.3 (CH₂), 26.3 (CH₂), 23.6 (CH₃), 22.4 (CH₃), 21.4 (CH₃). HRMS: m/z calcd for C₁₅H₂₄O₂Na [M + Na]: 259.1674; found: 259.1671.
(1 R ,2 S ,5 S ,6 R ,10 S ,12 S )-2-Isopropyl-5-methyl-15-oxatetracyclo[10.2.1.0 ^¹,5 .0 ^6,¹0 ]pentadecan-8-one (21) [α]_D ^²6 +72.8 (c 0.5, CHCl₃). IR (neat): ν_max = 2949, 2925, 2863, 1743 (C=O), 1470, 1404, 1380, 1177, 1100, 1080, 1036, 1001 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 4.54-4.44 (1 H, m, H-12), 2.86-2.72 (1 H, m), 2.64-2.45 (2 H, m), 2.43-2.22 (3 H, m), 2.20-2.11 (1 H, m), 2.10-1.94 (2 H, m), 1.86-1.72 (2 H, m), 1.70-1.44 (3 H, m), 1.39-1.22 (3 H, m), 1.20-1.06 (1 H, m), 1.00 (3 H, d, J = 6.1 Hz), 0.94 (3 H, s, tert CH₃), 0.85 (3 H, d, J = 6.6 Hz, CH ₃CHCH ₃). ^¹³C NMR (100 MHz, CDCl₃): δ = 218.3 (C, C=O), 94.2 (C, C-1), 76.7 (CH, C-12), 55.8 (CH), 52.5 (C), 46.3 (CH₂), 42.5 (CH₂), 42.3 (CH), 41.5 (CH₂), 40.4 (CH₂), 35.5 (CH₂), 34.8 (CH), 30.9 [CH, CH(CH₃)₂], 26.5 (CH₂), 26.1 (CH₂), 23.6 (CH₃), 21.5 (CH₃), 18.1 (CH₃). HRMS: m/z calcd for C₁₈H₂₈O₂Na [M + Na]: 299.1987; found: 299.1979.
(1 R ,2 S ,5 S ,6 R ,8 S ,10 S ,12 S )-2-Isopropyl-5-methyl-15-oxatetr acyclo[10.2.1.0 ^¹,5 .0 ^6,¹0 ]pentadec-8-yl 4-nitrobenzoate (23) Mp 91-93 ˚C (from 2-PrOH and hexane); [α]_D ^²6 +22.1 (c 0.7, CHCl₃). IR (neat): ν_max = 3113, 2951, 1724 (C=O), 1610, 1531, 1471, 1379, 1350, 1277, 1200, 1119, 1103, 1083, 1057, 1014, 992, 946, 915, 899, 874, 858, 838, 786, 721 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 8.28 (2 H, d, J = 8.8 Hz), 8.18 (2 H, d, J = 8.8 Hz, ArH), 5.52 (1 H, s, H-8), 4.54-4.38 (1 H, m, H-12), 2.84-2.59 (2 H, m), 2.47 (1 H, q, J = 10.1 Hz), 2.23-1.03 (15 H, m), 0.97 (3 H, s, tert CH₃), 1.00 (3 H, d, J = 6.1 Hz), 0.84 (3 H, d, J = 6.5 Hz, CH ₃CHCH ₃). ^¹³C NMR (100 MHz, CDCl₃): δ = 164.3 (C, OC=O), 150.4 (C), 136.2 (C), 130.6 (2 C, CH), 123.5 (2 C, CH), 94.1 (C, C-1), 79.6 (CH, C-12), 77.1 (CH, C-8), 57.0 (CH), 51.3 (C, C-5), 44.5 (CH), 41.4 (CH₂), 39.5 (CH₂), 38.9 (CH₂), 37.5 (CH₂), 36.1 (CH), 35.5 (CH₂), 31.0 [CH, CH(CH₃)₂], 27.3 (CH₂), 26.7 (CH₂), 23.2 (CH₃), 21.5 (CH₃), 20.6 (CH₃). HRMS: m/z calcd for C₂₅H₃₃NO₅Na [M + Na]: 450.2256; found: 450.2284.
Crystal Data for the PNB Ester 23
X-ray data were collected at 296 K on a SMART CCD-BRUKER diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods (SIR 92). Refinement was by full-matrix least-squares procedures on F2 using SHELXL-97. The nonhydrogen atoms were refined anisotropically whereas hydrogen atoms were refined isotropically. Molecular formula C₂₅H₃₃NO₅; MW = 427.52; colourless; crystal system: monoclinic; space group P21; cell parameters, a = 7.5430 (4) Å, b = 47.6237 (26) Å, c = 13.4683 (7) Å;
α = 90.00˚, β = 105.882 (3)˚, γ = 90.00˚, V = 4653.5 (42) Å^³, Z = 8, D _c = 1.220 g cm^-³, F(000) = 1840, µ = 0.084 mm^-¹. Total number of l.s. parameters = 1130, R1 = 0.0535 for 8130 F ₀ > 2σ(F ₀) and 0.1983 for all 20144 data. wR2 = 0.0816, GOF = 0.852, restrained GOF = 0.852 for all data. An ORTEP diagram is depicted in Figure [²] . Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC 797008). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.