Cycloadditions of N=S functions to cyclopentadienones

Article abstract of DOI:10.1016/S0040-4039(03)01646-0

Cyclopentadienones react with EtO₂CN=S=O and related N=S reagents to provide ready syntheses of 1,2,5-thiadiazolidines (8, 12, 17), diaminosulfanes (11, 13), an aminocyclopentenone (10) and the first unoxidised 1,2,3-oxathiazolidine (16), all i

Full text of DOI:10.1016/S0040-4039(03)01646-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6709–6711
Cycloadditions of NꢀS functions to cyclopentadienones
Hadjila Chabane,a Otto Meth-Cohn,a,* Charles W. Rees,b,* Andrew J. P. Whiteb and
David J. Williamsb
aChemistry Department, University of Sunderland, Sunderland SR1 3SD, UK
bChemistry Department, Imperial College London, London SW7 2AZ, UK
Received 2 June 2003; revised 20 June 2003; accepted 27 June 2003
Abstract—Cyclopentadienones react with EtO2CNꢀSꢀO and related NꢀS reagents to provide ready syntheses of 1,2,5-thiadiazo-
lidines (8, 12, 17), diaminosulfanes (11, 13), an aminocyclopentenone (10) and the first unoxidised 1,2,3-oxathiazolidine (16), all
in a mechanistically rational manner.
© 2003 Elsevier Ltd. All rights reserved.
Tetracyclone 1 is the classic reagent for the interception
of transient dienophiles, and cyclopentadienones are
widely used 4p components in Diels–Alder reactions.1
However, almost nothing is known of their reactions
with NꢀS containing dienophiles which are known to
react effectively with enes and dienes.2 Tetracyclone is
converted into tetraphenylpyrid-2-one 7 by warming
with trithiazyl trichloride (NSCl)3, which dissociates
into the monomeric thiazyl chloride, NSCl, a highly
reactive dienophile;3 this transformation is achieved
quantitatively and much more conveniently4 if the
NSCl is generated in situ from urethane, thionyl chlo-
ride and pyridine—the highly effective Katz reagent.5,6
With Katz reagent in boiling benzene (12 h) tetra-
cyclone 1 gave the pyrid-2-yl carbonate 6 (31%) and
thence the pyridone 7 (37%) but in boiling toluene (12
h) it gave 7 quantitatively. However, if the urethane
was replaced by EtO2CNSO 2, 1 was rapidly converted
into 7 (86%) in 3 h at room temperature (rt). There was
no reaction with 2 in the absence of thionyl chloride
and pyridine even on prolonged heating in toluene, but
on addition of pyridine (which catalyses the conversion
of 2 into the sulfur diimide 57) the pyridone 7 is again
formed together with a low yield of the formal [3+2]
cycloadduct 8 of 1 and 5. A much higher yield of 8
(81%) was formed directly from 5 in boiling toluene (12
h) and its structure was confirmed by X-ray crystallo-
graphy.8 Tetracyclone 1 is thus converted into the
pyridone under acidic conditions (SOCl2), and into the
cycloadduct 8 under neutral conditions, both in high
yield (Scheme 2).
We now report a diverse range of reactions of some
representative cyclopentadienones 1, 9 and 15 with
Katz reagent, which involves the intermediates 2, 3 and
4 (Scheme 1), and with preformed EtO2CNSO 2 and
the closely related N,N%-bis(ethoxycarbonyl) sulfur
diimide, EtO2CꢁNꢀSꢀNꢁCO2Et (5).
None of the analogous pyridones were formed from the
dimethyldiphenyl-cyclopentadienones 9 and 15. The
3,4-dimethyl compound 9 (Scheme 3) reacted slowly
with Katz reagent at rt to give a product shown (Fig. 1)
by X-ray crystallography8,9 to be the primary amine 10
(17%); this yield was increased to 54% by slowly adding
SOCl2 to a mixture of all the other reactants. An extra
equivalent of urethane was added to the Katz mixture
in the hope of generating the sulfur diimide 5 in situ,
and this mixture reacted with 9 at rt to give a minor red
oil 11 and a major colourless oil 12.
Scheme 1.
The cyclopentadienone structure of 11 was confirmed
by X-ray crystallography8 of 13 (Fig. 2), its Diels–Alder
adduct with DMAD (Scheme 3). The pattern of bond-
* Corresponding authors.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01646-0

6710
H. Chabane et al. / Tetrahedron Letters 44 (2003) 6709–6711
Product 12 is a formal [3+2] adduct of 9 and 5, and the
minor product was traced to the acid-catalysed conver-
sion of 9 into the small amount of the non-antiaromatic
isomer 14 with an exocyclic methylene group. Pure 14
gave the new cyclopentadienone 11 as the sole product
in up to 34% with all three NꢀS reagents. Preformed
sulfur diimide 5 converted 9 into the cycloadduct 12 in
high yield (79%) at rt in toluene; only a trace of 11 was
found since there was very little tautomerism of 9 under
these neutral conditions.
The 2,5-dimethyl analogue 15 exists as its Diels–Alder
dimer at rt and does not react with NꢀS reagents until
heated. The Katz mixture gave the formal [3+2] adduct
16 of 15 with EtO2CNSO 2 at 70°C (35%) and the same
product was formed in higher yield (74%) by brief (15
min) heating of neat 15 and 2 at 130°C. The sulfur
diimide 5, from Katz reagent with extra urethane, gave
the corresponding adduct 17 (65%) (Scheme 4).
Scheme 2.
We now suggest a rational and unifying mechanistic
scheme which accounts for these diverse products. Fol-
lowing Weinreb and Kresze’s detailed studies of simpler
dienes with NꢀS reagents,2 we propose that the reac-
tions all start by [4+2] cycloaddition of the 4p donor
cyclopentadienones with EtO2CꢁNꢀS+ꢁX where XꢀO−,
OSOCl, Cl and EtO2CN−; electrophilic catalysts are
represented by protons. The fate of the [4+2] adduct 18
(Scheme 5) depends critically upon its substituents and
Scheme 3.
Figure 2. The molecular structure of 13.
Figure 1. The molecular structure of 10.
ing within the –CH2N(CO2R)SN(CO2R)-fragment is
very similar to that observed in the only other struc-
turally characterised example of a molecule containing
this unusual fragment, dimethyl N,N%-(thio-bis-
(methyliminocarbonyloxy))-bis(ethanimidothiolate).10
Scheme 4.

H. Chabane et al. / Tetrahedron Letters 44 (2003) 6709–6711
6711
611 and finally gives pyridone 7. In contrast, the two
dimethylcyclopentadienones 9 and 15 gave no pyri-
dones, possibly because of reduced stability of the key
carbonium ions analogous to 20; furthermore the pres-
ence of a methyl group provides a lower energy path-
way for the conversion of 9 into the amine 10 (Scheme
6). The methylene isomer 14 of 9 is well set up to
undergo an ene reaction to give the bisaminosulfane 11
(Scheme 6). The [4+2] cycloaddition—2,3-sigmatropic
rearrangement sequence (Scheme 5) in the reaction of
the 2,5-dimethyl analogue 15 with 2 and 5 leads to the
observed adducts 16 and 17 (Scheme 4).
Thus we have shown that the readily available
cyclopentadienones react with some NꢀS reagents, also
readily available, to provide useful syntheses of some
new and rare structures. These are 1,2,5-thiadiazolidi-
nes and imidazolones derived therefrom, pyridones,
diaminosulfanes, an aminocyclopentenone, and the first
unoxidised 1,2,3-oxathiazolidine 16,12 and a mechanis-
tic framework is proposed to account for their
formation.
References
Scheme 5.
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deposit@ccdc.cam.ac.uk).
Scheme 6.
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In the absence of electrophiles such as thionyl chloride
the initial adducts undergo a thermal 2,3-sigmatropic
rearrangement (arrows in 18) to give the formal [3+2]
adducts 8, 12, 16 and 17. With tetracyclone 1 in the
presence of SOCl2 and pyridine, the adduct 18 can
undergo a faster acid-catalysed rearrangement (19“20)
via a stabilised 1,3-diphenylallylic carbonium ion 20, to
give N-ethoxcarbonylpyridone 21 which rearranges to