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Synlett 2010(20): 3015-3018  
DOI: 10.1055/s-0030-1259073
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

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;
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Publikationsverlauf

Received 18 October 2010
Publikationsdatum:
25. November 2010 (online)

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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

11

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

12

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): νmax = 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): νmax = 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): νmax = 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): νmax = 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): νmax = 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) ų, 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.