Synthesis of prelactone B

Article abstract of DOI:10.1055/s-1999-3421

Prelactone B (1a) and its isomer 1b are possible products of a truncated avermectin polyketide synthase. In order to make them available as reference compounds they were synthesized in a six-step synthesis with 56% overall yield.

Full text of DOI:10.1055/s-1999-3421

SHORT PAPER
401
Synthesis of Prelactone B
U. Hanefeld, A. M. Hooper, J. Staunton*
Department of Chemistry, University of Cambridge, LensÞeld Road, Cambridge CB2 1EW, UK
E-mail: js24@cus.cam.ac.uk
Received 28 November 1997, revised 30 June 1998
showed a typical a, a coupling between H-2a and H-3 (J
= 7.9 Hz) and between H-3 and H-4 (J = 8.2 Hz), while 1b
showed no couplings of comparable magnitude for either
H-3 or H-2, thus confirming the stereochemical assign-
ment of 1a.
Abstract: Prelactone B (1a) and its isomer 1b are possible products
of a truncated avermectin polyketide synthase. In order to make
them available as reference compounds they were synthesized in a
six-step synthesis with 56% overall yield.
Key words: prelactone B, Streptomyces griseus, avermectin,
polyketide synthase, aldol reaction
Recently it has been described that in the case of some
macrolides so called prelactones can be observed.1 Indeed
under certain conditions their production can be stimulat-
ed.2 During studies of the polyketide synthase (PKS) of
the avermectins and especially of its truncated version,
several δ-lactones (1a,b) were required as standards. In
the biosynthesis of avermectin3 a dehydration at C-23 can
take place. This leads to the avermectin 1 series. Without
dehydration the avermectin 2 series is obtained. In order
to clarify whether the responsible ketoreductase (KR) in
the PKS is selective and the dehydratase (DH) inefficient
or whether the KR is unselective and the DH accepts only
one of the possible products of the KR, both possible
triketide lactones (1a,b) were synthesized. One of these
lactones is a known natural product from Streptomyces
griseus (strain 2599 ana 18), Prelactone B (1a).1
The approach towards the target molecules had to allow a
flexible method for generating the chiral centre at C-3. It
was assumed that a nonselective strategy would be more
efficient, as both isomers were required and should be
readily separable.
The chiral centres at C-4 and C-5 of Prelactone B (1a) and
1b were constructed using Heathcock's approach.4 The al-
dol reaction led to the desired 4a in good yield but 28%
syn-isomer (4b) was also isolated. After protection of 4a
with tert-butyldimethylsilyl triflate5 the selective reduc-
tion of the amide moiety with lithium borohydride in the
presence of one equivalent of water6 proceeded smoothly.
The subsequent oxidation with DessÐMartin reagent7
gave aldehyde 7 in excellent yield. Since 7 was unstable,
it was treated, immediately after purification, with lithiat-
ed tert-butyl acetate. The aldol product 8 was obtained as
a mixture of diastereoisomers (1.23:1) in good yield. It
was deprotected and cyclised when treated with dilute
HCl. The lactones were isolated as a mixture (1a:1b =
1.23:1) in 56% yield over six steps. As expected 1a and 1b
were readily separable by reverse phase HPLC.
Both lactones proved to be very water-soluble and there-
fore only multiple extraction of the aqueous layer led to a
good recovery. This might cause problems when isolating
these compounds from natural sources.
The 1H and 13C NMR of prelactone B (1a) were in good
accordance with the published data. The optical rotation
was somewhat higher than reported and the synthetic sam-
ple was a solid. Presumably because of slight impurities,
the natural product did not crystallize. 8 Compound 1a
Solvents were purified and dried under standard conditions. All re-
actions involving air-sensitive reagents were performed under Ar.
1H and 13C NMR spectra were recorded on a Bruker AM-400
(400 MHz for 1H and 99.7 MHz for 13C) and were referenced inter-
Synthesis 1999, No. 3, 401Ð403 ISSN 0039-7881 © Thieme Stuttgart á New York

402
U. Hanefeld et al.
SHORT PAPER
nally on CHCl2 (1H, 7.27 ppm) and CHCl3 (13C, 77.5 ppm). J values
are given in Hz. IR. spectra were recorded on a Perkin-Elmer 1600
FT IR. Chromatography was performed using Merck Kieselgel 60,
particle size 0.040Ð0.063 mm. All compounds were pure by GC
and/or TLC. Mass spectra were recorded on a Kratos MS 890 dou-
ble focusing magnetic sector MS (EI, 70 eV) or on a Bruker FT-ICR
mass spectrometer (4.7 Tesla). Optical rotations were measured
with a Perkin-Elmer 241 polarimeter. For HPLC separations a Phe-
nomenex Prodigy 5u OD3, 100 •, 250 × 21.2 mm, 5 µ micron col-
umn was used. Mps were measured in an open capillary and are
uncorrected.
(2.59 g, 2.25 mL, 9.80 mmol) were added over a period of 5 min.
After stirring at r.t. for 20 min the reaction was quenched with H2O
(50 mL) and acidified (1 M HCl). The layers were separated and the
H2O was extracted with CH2Cl2 (2 × 50 mL). The combined organic
layers were washed with 1 M NaOH, dried (MgSO4) and evaporat-
ed to give 5 (2.75 g, 100%) as a colorless solid; mp 94Ð95¡C;
[a]D = Ð69.3 (c = 1.89 in CHCl3).
1H NMR (CDCl3): δ = 0.03 (s, 3 H, SiCH3), 0.08 (s, 3 H, SiCH3),
0.87 [s, 9 H, SiC(CH3)3], 0.90 (d, 3 H, J = 6.8 Hz, CH3), 0.91 (d, 3
H, J = 6.9 Hz, CH3), 0.92 (d, 3 H, J = 6.9 Hz, CH3), 1.15 (d, 3 H, J
= 6.7 Hz, 4-CH3), 1.75 (m, 1 H, H-4'), 4.04 (m, 2 H, H-2',H-3'), 4.75
(dq, 1 H, J = 7.1 Hz, 7.1, H-4), 5.63 (d, 1 H, J = 7.4 Hz, H-5), 7.34
(m, 5 H, C6H5).
(4S, 5R)-3-[(2'R, 3'R)-3'-Hydroxy-2', 4'-dimethylpentanoyl]-4-
methyl-5-phenyloxazolidin-2-one (4a)
13C NMR (CDCl3): δ = Ð4.36, Ð3.98, 13.78, 15.11, 16.58, 18.46,
20.76, 26.17 (3 C), 30.88, 43.84, 55.13, 76.22, 78.84, 125.73 (2 C),
128.73 (2 C), 128.78, 133.50, 152.69, 175.55.
To a solution of (4S, 5R)-3-propionyl-4-methyl-5-phenyloxazoli-
din-2-one (2) (2.50 g, 10.7 mmol) in Et2O (30 mL) freshly distilled,
neat dibutylboron triflate (5.4 mL, 21.4 mmol) and then diisopro-
pylethylamine (2.14 mL, 12.3 mmol) were added over a period of
8 min at 0¡C. After stirring at 0¡C for 30 min the mixture was
cooled to Ð78¡C and over a period of 10 min freshly distilled isobu-
tyraldehyde (3) (966 mg, 1.22 mL, 13.4 mmol) in Et2O (10 mL) was
added. The mixture was stirred at Ð78¡C for 30 min, diluted with
Et2O (15 mL), then quenched with (DL)-tartaric acid (8 g) and
stirred for 2 h at r.t. Water (40 mL) was added, the layers were sep-
arated and the water layer was extracted with Et2O (2 × 25 mL). The
combined organic layers were extracted with sat. NaHCO3 (2 × 25
mL). At 0¡C MeOH/30% aq H2O2 (40 mL, 3:1) was added slowly,
so that the temperature did not increase above 5¡C. The mixture
was stirred at 0¡C for 1 h and the volatiles were removed. The slurry
was extracted with Et2O (2 × 30 mL).9 The combined organic layers
were then extracted with sat. NaHCO3 (20 mL) and brine (20 mL)
and were dried (MgSO4). Chromatography (silica gel; petroleum
ether/EtOAc; 9:1) yielded 4a (2.05 g, 63%; mp 72Ð73¡C; [α]D = Ð
36.6 (c = 1.40 in CHCl3)) and 4b (0.93 g, 28%; mp 110¡C; [α]D =
Ð29.8 (c = 1.19 in CHCl3)) as colorless solids.
MS (FTICR): m/z (%) = 442.2 (M+ + Na, 100).
HRMS: m/z calc. for C23H37NNaO4Si: 442.23894; found: 442.23696.
(2S, 3R)-3-[(tert-Butyldimethylsilyl)oxy]-2, 4-dimethylpentanol
(6)
To a solution of 5 (2.5 g, 5.96 mmol) in anhyd Et2O (60 mL) H2O
(118 mg, 0.118 mL, 6.55 mmol) was added in one portion. At 0¡C
lithium borohydride (2 M in THF, 3.3 mL, 6.60 mmol) was added
over a period of 5 min and the mixture was then allowed to warm to
r.t. After stirring for 45 min the reaction was quenched with 1 M
NaOH (60 mL) and stirred until the layers were clear. After separa-
tion the water layer was extracted with Et2O (2 × 50 mL) and the
combined organic layers were washed with brine (50 mL) and dried
(Na2SO4). Chromatography (silica gel; petrol ether/ EtOAc; 9:1)
yielded 6 (1.37 g, 93%) as a colorless oil; [α]D = Ð8.35 (c = 1.39 in
CHCl3).
1H NMR (CDCl3): δ = 0.07 (s, 3 H, SiCH3), 0.10 (s, 3 H, SiCH3),
0.89 (d, 3 H, J = 6.5 Hz, CH3), 0.90 [s, 9 H, SiC(CH3)3], 0.91 (d, 3
H, J = 6.5 Hz, CH3), 0.96 (d, 3 H, J = 7.0 Hz, CH3), 1.83 (m, 2 H,
H-2, H-4), 2.68 (t, 1 H, J = 5.6 Hz, OH), 3.41 (t, 1 H, J = 4.9 Hz, H-
3), 3.57 (m, 1 H, H-1), 3.66 (m, 1 H, H-1).
13C NMR (CDCl3): δ = Ð3.96 (2 C), 16.58, 18.29, 18.45, 18.95,
26.10 (3 C), 33.12, 36.92, 66.04, 82.40.
MS (EI, 70 eV): m/z (%) = 246, (M+, 2), 203 (25), 187 (90), 147
(85), 133 (67), 119 (100).
HRMS: m/z calc. for C10H23O2Si (M+ Ð isopropyl): 203.1467;
found: 203.1468.
4a
1H NMR (CDCl3): δ = 0.90 (d, 3 H, J = 6.6 Hz, 5'-CH3), 0.95 (d,
3 H, J = 7.1 Hz, 2'-CH3), 0.97 (d, 3 H, J = 7.2 Hz, 4'-CH3), 1.20 (d,
3 H, J = 6.9 Hz, 4-CH3), 1.79 (m, 1 H, H-4'), 2.70 (d, 1 H, J =
9.6 Hz, OH), 3.45 (m, 1 H, H-3'), 4.06 (dq, 1 H, J = 7.1 Hz, 7.1, H-
2'), 4.76 (dq, 1 H, J = 6.8 Hz, 6.8, H-4), 5.66 (d, 1 H, J = 7.2 Hz, H-
5), 7.35 (m, 5 H, C6H5).
13C NMR (CDCl3): δ = 14.38, 15.11, 15.74, 19.91, 30.73, 40.42,
55.18, 78.99, 79.63, 125.66 (2 C), 128.77 (2 C), 128.86, 133.09,
153.38, 177.28.
MS (FTICR): m/z (%) = 328.2 (M+ + Na, 100).
(2R, 3R)-3-[(tert-Butyldimethylsilyl)oxy]-2, 4-dimethylpentanal
(7)
HRMS: m/z calc. for C17H23NNaO4: 328.15247; found: 328.15465.
4b
To a suspension of DessÐMartin reagent (1.0 g, 2.36 mmol) in
CH2Cl2 (50 mL) 5 (0.53 g, 2.15 mmol) in CH2Cl2 (10 mL) was add-
ed in one portion. The reaction was stirred at r.t. for 40 min and then
poured into 1 M NaOH/Et2O (50 mL/120 mL) and shaken. The lay-
ers were separated and the water layer was extracted with Et2O (100
mL). The combined organic layers were dried (Na2SO4). Chroma-
tography (silica gel; petrol ether/EtOAc; 95:5) gave 7 (0.52 g, 99%)
as a colorless, unstable oil. The aldehyde test, a typical yellow color
with 2,4-dinitrophenyl hydrazine, was positive. This material was
immediately used for the following aldol reaction.
1H NMR (CDCl3): δ = 0.90 (d, 3 H, J = 6.6 Hz, 5'-CH3), 0.94 (d, 3
H, J = 6.8 Hz, 2'-CH3), 1.04 (d, 3 H, J = 6.6 Hz, 4'-CH3), 1.20 (d, 3
H, J = 7.0 Hz, 4-CH3), 1.72 (m, 1 H, H-4'), 2.70 (d, 1 H, J = 3.5 Hz,
OH), 3.57 (ddd, 1 H, J = 8.6 Hz, 3.1, 3.1, H-3'), 4.05 (dq, 1 H, J =
7.0 Hz, 2.7, H-2'), 4.77 (dq, 1 H, J = 6.9 Hz, 6.9, H-4), 5.67 (d, 1 H,
J = 7.3 Hz, H-5), 7.35 (m, 5 H, C6H5).
13C NMR (CDCl3): δ = 10.01, 14.61, 18.88, 19.31, 30.91, 39.70,
54.90, 76.60, 78.90, 125.65 (2 C), 128.79 (2 C), 128.88, 133.16,
152.60, 177.63.
MS (FTICR): m/z (%) = 328.2 (M+ + Na, 100).
tert-Butyl (3SR, 4S, 5R)-5-[(tert-Butyldimethylsilyl)oxy]-3-hy-
droxy-4, 6-dimethylheptanoate (8)
HRMS: m/z calc. for C17H23NNaO4: 328.15247; found: 328.15013.
To diisopropylamine (505 mg, 0.70 mL, 5.0 mmol) in THF (80 mL)
BuLi (1.6 M in hexane, 3.13 mL, 5.0 mmol) was added over a peri-
od of 3 min at 0¡C. After stirring for 15 min tert-butyl acetate
(580 mg, 0.67 mL, 5.0 mmol) was added at Ð78¡C and stirred for
1 h. Then a solution of 7 (1.02 g, 4.2 mmol) in precooled THF
(4S, 5R)-3-{(2'R, 3'R)-3'-[(tert-Butyldimethylsilyl)oxy]-2', 4'-di-
methylpentanoyl}-4-methyl-5-phenyloxazolidin-2-one (5)
To 4a (2.0 g, 6.55 mmol) in CH2Cl2 (50 mL) diisopropylethylamine
(1.69 g, 2.28 mL, 13.10 mmol) and tert-butyldimethylsilyl triflate
Synthesis 1999, No. 3, 401–403 ISSN 0039-7881 © Thieme Stuttgart · New York

SHORT PAPER
Synthesis of Prelactone B
403
(15 mL) was added in one portion at Ð78¡C. The reaction was
stirred for 15 min and quenched with sat. aq NH4Cl (80 mL). The
layers were separated and the water layer was extracted with Et2O
(3 × 50 mL). The combined organic layers were dried (Na2SO4).
Chromatography (silica gel; petroleum ether/EtOAc; 95:5) yielded
8 (1.49 g, 98%) as a colorless oil.
1H NMR (CDCl3): δ = 0.04 (s, 3 H, SiCH3), 0.05 (s, 3 H, SiCH3),
0.06 (s, 3 H, SiCH3), 0.09 (s, 3 H, SiCH3), 0.82 (d, 3 H, J = 7.0 Hz,
CH3), 0.86 (d, 3 H, J = 6.8 Hz, CH3), 0.87 [s, 9 H, SiC(CH3)3], 0.88
(s, 9 H, SiC(CH3)3), 0.90 (d, 6 H, J = 6.0 Hz, 2 × CH3), 0.91 (d, 3 H,
J = 6.4 Hz, CH3), 0.94 (d, 3 H, J = 7.1 Hz, CH3), 1.41 [s, 9 H,
OC(CH3)3], 1.42 [s, 9 H, OC(CH3)3], 1.64 (m, 1 H, H-4), 1.78 (m, 2
H, H-4, H-6), 1.91 (m, 1 H, H-6), 2.23 (ddd, 2 H, J = 15.4 Hz, 3.5,
9.3, 2 × H-2), 2.45 (ddd, 2 H, J = 15.3 Hz, 5.5, 2.5, 2 × H-2), 3.44
(m, 3 H, OH, 2 × H-5), 3.51 (d, 1 H, J = 2.5 Hz, OH), 3.99 (ddd, 1
H, J = 7.5 Hz, 3.0, 3.0, H-3), 4.44 (m, 1 H, H-3).
H-6), 2.46 (dd, 1 H, J = 17.2 Hz, 7.9, H-2a), 2.65 (br s, 1 H, OH),
2.90 (dd, 1 H, J = 17.2 Hz, 5.8, H-2e), 3.75 (m, 2 H, H-3, H-5).
13C NMR (CDCl3): δ = 13.65, 14.09, 20.04, 28.92, 38.95, 39.05,
69.71, 86.43, 171.43.
IR (nujol): ν = 3453 (OH), 2360, 1702 (C=O), 1010 cmÐ1.
MS (FTICR): m/z (%) = 195.1, (M+ + Na, 100).
HRMS: m/z calc. for C9H16NaO3: 195.09971; found: 195.10036.
1b
Mp 130Ð132¡C; [α]D = 33.1 (c = 0.99 in MeOH).
1H NMR (CDCl3): δ = 0.88 (d, 3 H, J = 6.8 Hz, CH3), 1.03 (d, 3 H,
J = 6.9 Hz, CH3), 1.10 (d, 3 H, J = 7.0 Hz, CH3), 1.87 (m, 2 H, H-4,
H-6), 2.37 (br s, 1 H, OH), 2.66 (d, 1 H, J = 4.0 Hz, H-2), 2.75 (d, 1
H, J = 2.9 Hz, H-2), 4.06, (m, 1 H, H-3), 4.31 (dd, 1 H, J = 2.0 Hz,
10.5, H-5).
13C NMR (CDCl3): δ = 13.50, 14.04, 19.88, 28.86, 34.71, 39.36,
67.99, 84.15, 170.91.
MS (FTICR): m/z (%) = 195.1, (M+ + Na, 100).
13C NMR (CDCl3): δ = Ð4.11, Ð4.02, Ð3.82, Ð3.69, 11.77, 13.85,
17.72, 18.30, 18.40, 18.78, 19.69, 19.94, 26.07 (3 C), 26.21 (3 C),
28.09 (6 C), 32.03, 32.28, 38.21, 40.06, 41.18, 42.07, 67.49, 69.92,
80.04, 80.60, 80.73, 82.70, 171.59, 172.47.
HRMS: m/z calc. for C9H16NaO3: 195.09971; found: 195.10133.
MS (FTICR): m/z (%) = 383.3 (M+ + Na, 100).
HRMS: m/z calc. for C19H40NaO4Si: 383.25939; found: 383.26190.
(1) Bindseil, K. U.; Zeeck, A. Helv. Chim. Acta 1993, 76, 150.
(2) Boddien, C.; Gerber-Nolte, J.; Zeeck, A. Liebigs Ann. 1996,
1381.
Prelactone B (1a), and (3S, 4S, 5R)-3-Hydroxy-4,6-dimethyl-
heptanoic Acid-δ-lactone (1b)
Conc. HCl (1 mL) was added dropwise over a period of 2 min to a
solution of 8 (200 mg, 0.55 mmol) in THF (8 mL) and H2O (2 mL)
and stirred at r.t. for 7 days. The solvents were removed in vacuo to
yield a mixture of 1a and 1b (1.23:1; 93 mg, 99%) as a colorless sol-
id. The mixture was separated by HPLC. Flow rate: 20 mL/min.
Gradient: Start with 35% MeOH and 65% H2O, go to 51% MeOH
and 49% H2O over 16 min. Retention time: 1a 11.14 min and 1b
12.10 min.
(3) Dutton, C. J.; Hooper, A. M.; Leadlay, P. F.; Staunton, J. Te-
trahedron Lett. 1994, 35, 327.
(4) Raimundo, B. C.; Heathcock, C. H. Synlett 1995, 1213.
(5) Corey, E. J.; Cho, H.; RŸcker, C.; Hua, D. H. Tetrahedron
Lett. 1981, 22,3455.
(6) Penning, T. D.; Djuric, S. W.; Haack, R. A.; Kalish, V. J.; Mi-
yashiro, J. M.;Rowell, B. W.; Yu, S. S. Synth. Commun. 1990,
20, 307.
(7) Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277.
Ireland, R. E.; Liu, L. J. Org. Chem. 1993, 58, 2899.
Meyer, S. D.; Schreiber, S. L. J. Org. Chem. 1994, 59, 7549.
(8) The natural product did solidify later. A. Zeeck, personal
communication.
(9) It was found in later experiments that under these conditions
peroxides were formed in the Et2O layer. These can be de-
stroyed with 2 M aq Na2SO3.
1a
Mp 97Ð98¡C; [α]D = 62.1 (c = 1.72 in MeOH).
IR (nujol): ν = 3466 (OH), 2360, 2341, 1716 (C=O), 998 cmÐ1.
1H NMR (CDCl3): δ = 0.90 (d, 3 H, J = 6.8 Hz, 6-CH3), 1.05 (d, 3
H, J = 6.7 Hz, 4-CH3), 1.07 (d, 3 H, J = 6.8 Hz, 7-CH3), 1.73 (ddq,
1 H, J = 10.4 Hz, 8.2, 6.7, H-4), 1.97 (dsept., 1 H, J = 2.1 Hz, 6.9,
Synthesis 1999, No. 3, 401Ð403 ISSN 0039-7881 © Thieme Stuttgart á New York