A new mild PTSA-catalyzed method for sulfate ester hydrolysis and acid-catalyzed rearrangement of 12-acetyl-diene-11-ol tetracyclic triterpenoids involving an angular methyl migration

Article abstract of DOI:10.1016/S0040-4039(00)01190-4

Tetracyclic triterpenoids containing the 12-acetyl-Δ^8,14-diene-ll-ol moiety undergo a series of acid-catalyzed rearrangements. The rearrangement products have been characterized, plausible mechanisms for the rearrangement have been elucidated and conditions have been developed to give high yields of the rearrangement products. A new and general PTSA·H₂O and PPTS-catalyzed sulfate hydrolysis method has been developed. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01190-4

Tetrahedron Letters 41 (2000) 6973±6976
A new mild PTSA-catalyzed method for sulfate ester
hydrolysis and acid-catalyzed rearrangement of
12-acetyl-diene-11-ol tetracyclic triterpenoids involving
an angular methyl migration
Sheo B. Singh
Merck Research Laboratories, mail code RY80Y-355, PO Box 2000, Rahway, NJ 07065, USA
Received 27 April 2000; revised 11 July 2000; accepted 12 July 2000
Abstract
Tetracyclic triterpenoids containing the 12-acetyl-Á8,14-diene-11-ol moiety undergo a series of acid-
catalyzed rearrangements. The rearrangement products have been characterized, plausible mechanisms for
the rearrangement have been elucidated and conditions have been developed to give high yields of the
.
rearrangement products. A new and general PTSA H2O and PPTS-catalyzed sulfate hydrolysis method has
been developed. # 2000 Elsevier Science Ltd. All rights reserved.
The sulfated tetracyclic triterpenoid microbial metabolite 1 has been shown to possess a variety
of biological activities. It was ®rst discovered in our laboratories as an inhibitor of elastase
(IC50=2.4 mM),1 a protease which represents a potential therapeutic target for in¯ammatory and
degenerative diseases. Subsequently, it was reported as an inhibitor of rhinovirus 3C protease
(IC50=80 mM)2 and cholesteryl ester transfer protein (IC50=13.8 mM).3 The biological activities
of compound 1 prompted us to investigate its chemistry for a structure±activity relationship
(SAR). This required the conversion of compound 1 to compound 2a by sulfate hydrolysis.
There are three main methods known for the hydrolysis of sulfate esters: (i) solvolysis by
heating in dioxane;4 (ii) mineral-acid-catalyzed hydrolysis;5 and (iii) heating of the ester in a
mixture of dioxane and pyridine.6 The ®rst method appeared attractive because of its simplicity
and became the method of choice initially. In this method compound 1 was heated in dioxane at
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01190-4

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50±60ꢀC for 30 min to give a good yield (ꢁ50±60%) of the desired compound 2a.1a However,
when the reaction was repeated it gave irreproducible results particularly on a larger scale and
consistently a€orded a mixture of rearrangement products 3±87 in 2±5% yield. Sulfuric acid-
catalyzed reaction also produced these compounds as the main products. Structures of these
compounds (3±8) and plausible mechanisms for the rearrangements are shown in Figs. 1 and 2.
Figure 1. Structures and plausible mechanism of rearrangements
Figure 2. Structures and plausible mechanism of rearrangements
In the third hydrolysis method, heating of compound 1 in a 1:1 mixture of dioxane and
pyridine gave a brownish reaction mixture mostly containing compound 2a along with low levels
of several uncharacterized products as detected by HPLC. However, none of the methods gave
satisfactory results on a gram scale and a new method was urgently needed.
In order to probe for milder methods of hydrolysis and to ®nd an ecient method for the
preparation of the rearrangement products, a set of parallel experiments was designed where 1 or
.
2a was reacted with either mineral acids (HCl or H2SO4) or organic acid (PTSA H2O) in various
aprotic and protic solvents (dioxane, THF, CH3CN, CH3OH and H2O) that led to the
identi®cation of the following conditions for both processes.
Controlled rearrangements: The reaction of either compound 1 or 2a in neat methanol with
.
H2SO4 or PTSA H2O at ambient temperature in less than 30 min produced compounds 3, 4, 5
and 6 in a ratio of 45, 45, 5, 5 (combined 90% yield), respectively. Addition of co-solvents (10 to
90%, v/v) such as dioxane and water in methanol did not have any detrimental e€ect on the
H2SO4-catalyzed reaction other than slowing the rate of reaction. However, similar dilution had a
.
signi®cant impact on the reactivity of compound 2a in the PTSA H2O-catalyzed reaction

6975
including complete recovery of the starting substrate. Accordingly, the reaction of the 12-hydroxy
.
compound 2b with PTSA H2O in CH3OH produced exclusively compounds 3 and 4 and therefore,
these reactions provide a new method for ecient synthesis of C-ring aromatic compounds from
this expanding series of natural products.
Ecient and decomposition free sulfate hydrolysis of compound 1: The aforementioned observation
.
of the sulfate hydrolysis and stability of compound 2a in dioxane containing PTSA H2O led to
the development of the following ecient method for the conversion of compound 1 to 2a. The
.
reaction of 1 with 10±100 mol% of PTSA H2O in dioxane exclusively produced compound 2a in
5±30 min (depending on the scale) without any rearrangement products.8 The reaction is
consistently reproducible and amenable to larger scale synthesis as demonstrated by the conver-
sion of 29 g of 1 to 23 g of 2a in 92% yield. During this study it was observed that the free acid
form of compound 1 undergoes signi®cant decomposition upon storage, which can be prevented
by conversion to a sodium salt.
.
General sulfate hydrolysis: This PTSA H2O-catalyzed method of the sulfate hydrolysis is
general as exempli®ed by the hydrolysis of the sulfates 9a±13a (Fig. 3, Table 1) to their
corresponding hydroxy derivatives 9b±13b, respectively. The rate of the hydrolysis of these com-
pounds varied due to the solubility of the sulfated salts in aprotic solvents. For example, the
.
reactions of compounds 9a±11a in dioxane with 50 mol% of PTSA H2O in less than 30 min gave
over 90% yield of cholesterol (9b), cholestanol (10b) and pregenalone (11b), respectively.
However, due to the lack of solubility of estradiol-potassium-bis-sulfate (12a) and 4-methyl-
umbelliferone potassium sulfate (13a) in dioxane or THF, these substrates required the addition
of water for better solubility. The hydrolysis of these sulfates then required longer reaction time
and elevated temperature as exempli®ed below. The reaction of 12a in 1:1 dioxane:water with 50
.
mol% of PTSA H2O required reaction for 72 h at ambient temperature to selectively produce the
mono-sulfated estradiol 12b, which, upon further heating at 60ꢀC, a€orded diol 12c.
Alternatively, the reaction in 1:1 methanol:water with heating at 80ꢀC for 70 h also exclusively
a€orded 12b. The hydrolysis of the second sulfate group in this reaction was dicult and required
.
an additional equivalent of PTSA H2O followed by further heating. Likewise, the reaction of 13a
in 3:1 methanol:water with heating at 60ꢀC for 16 h gave 4-methyl-umbelliferone 13b. For a head-
to-head comparison of di€erent hydrolytic methods, sulfate hydrolysis reactions of compounds
10a and 11a were repeated using H2SO4 and solvolysis in dioxane. All three reactions gave
identical results except that the rate of the hydrolysis in neat dioxane was slightly slower and
required heating at 80ꢀC. PPTS is also an e€ective hydrolytic agent but required heating.
.
In conclusion, a PTSA H2O-catalyzed versatile and mild method for the hydrolysis of sulfate
esters, either as the free acid or salt form, has been developed.9 In contrast to other methods, this
method can be used for the hydrolysis of sulfates possessing heat and mineral acids sensitive
Figure 3. Structures of sulfates and hydrolyzed products

6976
Table 1
PTSA H2O-catalyzed hydrolysis of sulfates
.
molecular structures. In addition, the products of the acid-catalyzed rearrangement of compound
2a have been characterized and several plausible mechanisms have been proposed. Ecient
methods for the rearrangements have been developed that provide the possibility for the ecient
synthesis of C-ring aromatic compounds of this expanding series of natural products.
Acknowledgements
The author is indebted to Dr. Z. Guan and Ms. Deborah Zink for providing mass spectral
analyses.
References
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7. Purity of compounds was veri®ed by reverse phase HPLC and their structures were fully elucidated by 1D and 2D
NMR spectroscopy and mass spectrometry including MS/MS measurements.
.
8. In a typical experiment, to a solution of compound 1 (29 g, 0.047 mol) in dioxane (300 mL) was added PTSA H2O
(2.9 g, 0.015 mol) and the solution was stirred at room temperature. After 30 min it was quenched by addition of a
10% aqueous solution of NaHCO3, extracted with ethyl acetate (3Â1.5 L), and the organic layer was separated,
washed with water (2Â1 L), dried (Na2SO4), concentrated and chromatographed on a silica gel column to give
compound 2a (23 g, 92%).
9. The sulfate group of a sulfated hexaglycoside was hydrolysed by this method without a€ecting the glycosidic
linkages. Due to the interest of the biological activity of this new glycoside, the structure cannot be disclosed at
this point and will be reported elsewhere in due course.