3802
J . Am. Chem. Soc. 1987, 109. 3802-3804
Figure I. ORTEP drawing ofdithiatopazine (I).Bond distances of l.2dithietanc ryslem (A): CSa-Cl2a = 1.571 (h);CI2a-S15 = 1.844 (4); SlS-SI6 = 2.084 (2): S16-CSa = 1.881 (4). Bond angles (deg): C5eC12a-SIS =97.4(3):C12a-S15-S16 = 82.2(l):SIS-Slh-C5a = 80.7 (I):Sl6-CSa-Cl2a = 96.8 ( 3 ) . Dihedral angle (dcg) bctween planesCI2a-S IS-SI6 andCSa-Slh-SI5 = 11.0(4).
(iii) xylene. I 6 0 " C , 88%. Furthcrmorc. cpiculfidc V w a ~ transformed to the spiro ketal-ketonc V I upon cxposurc to mCPBA-H,O in CH,CI, (55%). I n order to confirm the 1,2-dithictanc itructurc o f compound Iand to determine some of its molecular parameters. a n X-ray crystallographic analysis was undertaken. Compound I crystallim in the orthorhombic space group Phca with a = 9.549 (3) A. b = 12.024 (3) A. c = 28.659 (7) A, L' = 3290.7 A', and p(calcd) = 1.391 g cm-' for z = 8. The structure was solved by direct methods and Fourier techniques and refined by full-matrix least squares to R , = 0.049 and R, = 0.057using 1605 unique, observed ( I > 3n) reflections. Figure 1 shows an ORTEP representation or the moleculc and includes a numbcr of bond lengths and bond angles. O f special interest are the rather long SIS-Slh (2.OX4 (2) A] and C4a-CI4a bond [ I S 7 1 (6) A] as well as thc rcmarkably small anglcsat sulfur [S16-S15-CIZa = 82.2 (I)"and SIS-Slh-C5a = 80.7 (I)"]. The dihedral angle between planes S15-S16-C5a and S16-SI5-Cl2a i s notably small. 11.0 (4)O. and suggcsts considerable rcpulsivc overlap of thc lunc pairs of electrons on the two sulfurs. Yet, the 1,2-dithietane moiety in this system is phenomenally stable. Further exploration o f the physical, chemical. and biological properties ofdithiatopazine (I)and the design, synthesis. and study of other 1.2-dithietane systems are currently being pursued in these laboratories.'4
properties and was confirmed by an X-ray crystallographic analysis (vide infra). Thus, 1 exhibited the following spectra data: UV-vis (hexane),,A,, 213 (c 4074). 426 nm ( 6 102):l2 I R (CCI,) urnax2940, 2840, 1450, 1275. 1160, 1094, 1052. 965 cm-I; 'H N M R (500 M H z , CDCI,) 6 3.98 (m. I H, C H O closest and syn to S-S bridge). 3.90 (m. 1 H, CH,O, equatorial), 3.82 (m, 1 H. CH,O, equatorial), 3.35-3.12 (m. 5 H,CHO, CH,O), 3.00 (m. I H. CHJ. 2.54 (m, 1 H, CH,), 2.34 (m, I H, CHI), 2.15 (m. I H. CH,), 2.03-1.40 (m. 12 H, CH,); 'IC N M R (125 MHz. CDCI,) Acknowledgment. We express thanks to Drs. George Furst and 6 104.02, 102.18,82.69,80.71,77.69,76.17,67.82,67.14,39.04,John Dykins ofthis Department for their superb N M R and mass 34.43, 31.25.30.86, 29.40.28.99. 25.80. 25.42: H R M S (CI) calcd spectroscopic assistance and useful commments. We also thank for C,H,O,S, H 345.1194, found 345.1233 (M H). Professors E. R. Thornton, University of Pennsylvania, E. Block. The chemistry of I is quite intriguing and has already led to University of New York, Albany, K. Steliou. University of a number of novel systems as indicated in Scheme 1. Thus, Montreal, and Dr. J. P. Snyder, G. D.Searle, for stimulating irradiation of 1 (toluene. Hanovia U V lamp, 1 h) at ambient discussions concerning this work. This work was financially temperature resulted in extrusion of sulfur and the clean formation supported by the National Institutes of Health. Merck Sharp and of olefin 111 (90%). Extrusion o f sulfur from 1 and generation Dohmc, Hoffmann-La Roche, and Smith Kline Beckman. of 111 (95%) was also observed upon thermolysis of I (neat, or Supplementary Material Available: Spectroscopic and analytical in xylene solution, 140 "C). Reduction of 1 with n-Bu,SnH-AIBN data for compounds 11-VI and tables of refined atomic positional also produced 111 in high yield (97%). Treatment of dithiatopazine and thermal parameters and bond distances and angles for com( I ) with PPh, (CH,CI,. 25 "C) led smoothly to the fascinating pounds I and V I (7 pages). Ordering information i s given on any compounds I V (46%) and V " (45%) by abstraction of one ofthe current masthead page. sulfur atoms.13 The structure of the spiro ketal-thioketone I V was based on its spectral data, particularly its "C N M R spectrum [I25 MHr, benzene-d,, 6 262.56 (C=S) and 106.32 (0-C-O)]. (14) All new camaunds exhibited satisfactory spectral and analytical and is smooth conversion to spiro ketal-ketone V I by ozonolysis andjar exact mass data. Yields refer to spectroscopically and chramatographically homogeneous materials. (85%) (Structure V I ) and was confirmed by X-ray crystallographic analysis (see ORTEP drawing. Scheme I ) . The structure of the surprisingly stable episulfide V was assigned on the basis of i t s spectral data, particularly its " C N M R spectrum [I25 M H z , CDCI,, 6 93.28 (0-C-S) and 86.91 (0-C-S)], and its chemistry. Thus. V suffered loss of sulfur and transformation to olefin 111 Peptide Synthesis Catalyzed by Lipases in Anhydrous by any one of the following procedures: (i) n-Bu,SnH-AIBN Organic Solvents calalyst. toluene, II O "C, 93% (ii) (EtO),P. toluene, 110 OC, 94%:
+
+
(II)TLC analysis of the melt revealed milia1decommition to olefin 111. and presumably sulfur. (I2) Slow decomposition to olefin 111, and presumably sulfur. was observed during the UV-vir measurements as indicated by decrease of absorbance with time. The reported < vducs. therefore. must be minima. (13) X h c m e IIshows possible mechanistic pathways for these intemting transformations. Thus initial attack by Ph,P on Imay in principle rcsull in
Alexey L. Margolin and Alexander M. Klibanov* Department of Applied Biological Sciences Massachusetts Institute of Technoloyy Cambridge, Massach usetts 02 I 3 9 Receiued February 12. 1987
Scheme II
PP",
i" \ / I"
two isomeric species VI1 depending on the rcgiochemirtry of the attack. These rlcreochemically distinct species may prefer different reaction pathways depending on sterewlectronic effects and may lead to I V (path a) or oxonium species V l l l (path b). Oxonium species V l l l may then collapse to episulfidc V (path c) or rearrange to thioketone I V (path d). Calculations, molecular modeling, and further experiments are expected to provide further mechanistic
information.
One of the bottlenecks of the rapidly growing field of peptide research i s the shortage of general methodologies for facile preparation o f a wide range of diverse peptide structures.' Enzymatic, namely, protease-catalyzed. synthesis i s emerging as a method of choice for the production of short peptides due to i t s mild reaction conditions, absence of racemi7ation. minimal protection and activation requirements. and inherent rcgio- xnd stereoselectivities.z However, peptide bond formation catalyzed
New Y&k. 1970-1982: Vols. 1-6. Gross. E.: Mcicnhbfcr. J . The Pq1ide.v: AmIy$is, Synthesir. Biology; Academic: New York. 1979-1985: Vols. 1-7.
0002-7863/87/1509-3802$01.50/00 1987 American Chemical Society
J . Am. Chem. SOC.,Vol. 109, No. 12, 1987
Communications to the Editor
3803
Table I. Peptide Synthesis Catalyzed by Porcine Pancreatic Lipase in Organic Solvents" substrates, amino acid fragment carboxy terminal
amino terminal
solventb
N-Ac-L-Phe-OEtCI N-Ac-L-Phe-OEt N-Ac-L-Tyr-OEtCI A'-Ac-L-Tyr-OEt N-Ac-L-Met-OEtC1 N-Ac-~-Phe-0EtCl N-Ac-L-Phe-OEtC1 N-Ac-L-Phe-OEtCI IV-Ac-L-Phe-OEtC1 N-Ac-i-Phe-OEtC1 N-Ac-L-Phe-OEtCI N-Ac-L-Phe-OEtCI
L-Leu-", L-Leu-"; L-L~u-NH, L-Leu-", L-L~u-NH, L-AI~-NH, L-Val-", r-Phe-NH, o-Leu-", ~-Ala-oMe o-Ala-OMe L-Leu-NHNaph"
toluene toluene tetrahydrofuran tetrahydrofuran toluene toluene toluene toluene toluene toluene toluene toluene
product' N-Ac-L-Phe-L-Leu-", N-Ac-r-Phe-r-Leu-NH;e N-Ac-L-Tyr-L-Leu-NHi 12'-Ac-L-Tyr-L-Leu-", N-Ac-L-Met-L-Leu-NH,g ~~-Ac-L-Phe-L-Ala-NH,h N-Ac-L-Phe-L-Val-",' A'-Ac-(L-Phe),-NHi N-Ac-L-Phe-D-Leu-",' ,Y-Ac-L-Phe-L-Ala-OMe' N -Ac-r-Phe-o-Ala-OMe'
A'-Ac-L-Phe-L-Leu-NHNaph"
initial rate,d nmol/min (reaction half-time,' days)
isolated yield of the product, %
12.8 (1.4) 4.2 (4.2) 7.5 (2.3) 4.2 (4.2) 4.3 (4.0) 11.9 (1.5) 11.8 (1.5) 4.9 (3.5) 9.8 (2.0) 3.8 (4.5) 3.7 (4.5) 2.1 (8.3)
83 78 76 67 57 82 86 48 76
I I 51
all experiments (except for the one depicted in the first line, which is described in the text), 2.5 mmol of both substrates was dissolved in either toluene or tetrahydrofuran, followed by addition of 5 g8 of porcine pancreatic lipase. The suspension* was shaken at 45 " C and 250 rpm for the period of time after which no more formation of the alcohol product (Cl-EtOH or E t O H ) was detected by gas chromatography, and then the dipeptide synthesized was isolated following the general outline given in the text. I n no case was any peptide formation detected (by GC, HPLC, or T L C ) in the absence of the enzyme or in the presence of lipase irreversibly preinactivated by diethyl p-nitrophenyl phosphate.' Both solvents were of analytical grade and subjected to no additional purification prior to use apart from drying by storing in the presence of 3-,& molecular sieves (Linde).
