The synthesis and reactivity of new 2-(N,N-diisopropylamino)-3-methylsulfonyl-1,3,2-benzoxazaphospholes. The utility of the 5-chloro analogue in the one-pot synthesis of oligothiophosphates: [ApSppA, ApSpppA, ppp5′A2′pS5′A, m7GpSppA, ApSpppp, ApsPP]

Article abstract of DOI:10.1016/0040-4020(95)00031-3

The synthesis of 2-(N,N-diisopropylamino)-2,3-dihydro-3-methylsulfonyl-1,3,2-benzoxazaphospholes 22, 23 and 24 is reported. Their reactivities have been investigated using a variety of acid catalyst under conditions normally employed in phosphoramidite chemistry for oligonucleotide synthesis. The rate (k) of activation of 22, 23 and 24 by acid catalysis with N-methylanilinium hydrochloride (MAC) to their protonated species 25A, 26A and 27A, respectively, (Scheme 2) has been estimated to be 6.813 × 10^-7 mol^-1 min^-1, 1.237 × 10^-6 mol^-1 min^-1 and 1.972 × 10^-7 mol^-1 min^-1 at 18°C with a ratio of the rates as 0.56:1:0.16. The 5-chlorobenzoxazaphosphole 23 selectively activated by MAC gave the intermediate 2-(N-methylanilinium)-5-chlorobenzoxazaphosphole 26A ( 90% by NMR), which was then reacted with alcohols (nucleosides) to generate the reactive 2-alkoxybenzoxazaphosphole 30 (Scheme 4) or 33-35 (Scheme 5) ( 70% by NMR). We have then shown that 33-35 (Scheme 5) react with binucleophilic reagents, ADP, ATP or pyrophosphate, to generate the corresponding P¹-alkoxycyclometatriphosphite intermediate 36, 37, 50, 52 or 54. These intermediates were then sulfurised to form the P¹-alkoxy-1-thiocyclotriphosphate intermediates 38, 39, 51, 53 and 55, which were ring-opened by hydrolysis. Thus these steps constituted a one-pot multicomponent reaction (MCR) leading to the synthesis of R_P and S_P mixtures of each of the mono-thioanalogues of naturally-occurring oligophosphates: Ap_SppA (43) (10%), Ap_SpppA (45) (19%), ppp5′A2′p_S5′A (46) (24%), the cap structure m⁷Gp_SppA (47) (3%), Ap_Spppp (42) (23%), Ap_Spp (41) (33%) and Ap_Sp (40) (7%). The reaction of putative 34 and ADP gave the desired 43 (10%) along with pp5′A2′p_S 5′A (44) (5%). The reaction sequences from 34 and ATP gave 45 (19%) and ppp5′A2′p_S5′A (46) (24%). The proposed reaction mechanism for the synthesis of 43, 45 and 47 proceeds via the corresponding (dinucleoside 5′)-cyclometatriphosphite intermediates 50, 54, 52 and the (dinucleoside 5′)-1-thiocyclotriphosphate intermediates 51, 55, 53. The existence of these cyclic P(III) and P(V) intermediates were supported by ³¹P- and ¹H-NMR spectroscopy. We here also demonstrate that 5-chlorobenzoxazaphosphole 23 can be used to synthesise a protected ribonucleoside 2′,3′-Cylic phosphorothioate block 28, a protected ribonucleoside 3′-(O-(4-chloro-2-mesylsulfonamido)phenyl phosphorothioate diester block 29 and bis(2-deoxyadenosine-5′)-thymidine-3′-monophosphate 32. The correct coupling of the nucleoside residues to the oligophosphate chain in 43, 45 & 47 has unequivocally been assessed by 2D ³¹P^-31P and ¹H^-31P correlation spectroscopy.