Chemical Conversion of Ryanodol to Ryanodine

Abstract

Ryanodine (1) is a plant-derived natural product with powerful pharmacological and insecticidal action, and is a potent modulator of intracellular calcium release channels. Compound 1 possesses a 1H-pyrrole-2-carboxylate ester at the C3-position of heptahydroxylated terpenoid ryanodol (2). Whereas 2 was readily obtained from 1 by basic hydrolysis, 1 has never been synthesized from 2, due to the extreme difficulty in selectively introducing the bulky pyrrole moiety at the severely hindered C3-hydroxyl group of heptaol 2. Here we report chemical conversion of 2 to 1 for the first time. The derivatization was realized through the use of a new protective group strategy and the application of on-site construction of the pyrrole-2-carboxylate ester from the glycine ester and 1,3-bis(dimethylamino)allylium tetrafluoroborate.Key words acylation; protecting group; regioselectivity; ryanodine; terpenoid Ryanodine (1, Chart 1) has been isolated from the wood of Ryania speciosa VAHL and shown to possess potent insecticidal and pharmacological actions.1) Compound 1 binds specifically to a high-conductance intracellular calcium channel known as the ryanodine receptor (RyR), altering its conductance.2,3) Since the RyR plays an important role in many biological processes and malfunctions in its action have been linked to skeletal and cardiac diseases, 4-7) 1 has been used as a biological probe for investigations of RyR function, and is regarded as a strong candidate in the development of therapeutic agents for various diseases. [8][9][10] In 1968, the structure of ryanodine (1) was determined to be the ester between 1H-pyrrole-2-carboxylic acid (3) and the C3-hydroxyl group of ryanodol (2). [11][12][13][14] Compound 2 is a diterpenoid containing the exceedingly complex five fused rings (ABCDE-rings) with eleven contiguous stereocenters. Interestingly, the affinity of 2 toward RyRs of vertebrate skeletal muscle is 1700 times weaker than that of 1 (K D =7 nM), [15][16][17] demonstrating that the pyrrole group is a primary determinant for strong interactions of 1. Because of its important biological activities and intriguing structural features, many studies attempting the chemical construction of 1 has been carried out, which resulted in the total syntheses of ryanodol (2) by the Deslongchamps group in 1979 18,19) and by our laboratory in 2014. [20][21][22] In 1951, before the complete structure of 1 was known, Wiesner and colleagues found that basic hydrolysis of ryanodine (1) generated 2 and 3.11) However, the reverse reaction has not been realized even to date. The difficulty of the transformation arises from the need to control the regioselectivity of the four tertiary (C2, 4, 6, 12), two secondary (C3, 10) and one acetalic (C15) alcohols within heptaol 2 and to accommodate the bulky pyrrole moiety at the hindered C3-hydroxyl group. The computer-generated 23) structure of 2 revealed that its axially oriented C3-OH was shielded by the proximal C14-hydrogen atom and by the C19-, 20-methyl groups (Fig. 1). In fact, direct esteri...

Chemical Synthesis of (+)-Ryanodine and (+)-20-Deoxyspiganthine

“…Although this compound could not be directly coupled with pyrrole-2-carboxylic acid (or its derivatives), it could be esterified with N , N -di( tert -butoxycarbonyl)glycine, and elaborated in an additional 5 steps to 1 . 25 In sum, the conversion of 4 to 1 required ten synthetic steps. Alternatively, Inoue and co-workers demonstrated that a synthetic precursor to 4 could be advanced directly to 1 , allowing access to ryanodine in 42 linear steps.…”

“…29 The synthesis was enabled by the expedient, gram scale preparation of tetracycle 6 , which can be oxidized using SeO 2 , then triflated, to give either of two oxidation products, 7 or 8 (Figure 2A). In considering how to translate our synthesis of 4 to a synthesis of 1 , our guiding strategic objective was to incorporate the pyrrole-2-carboxylate ester directly—rather than building it from a glycinate ester 25,28 —by an esterification reaction in the final stage of the synthesis. Given the challenges encountered by others when trying to acylate 4 , or appropriately protected derivatives of 4 , we decided to target the acylation of a protected derivative of (+)-anhydroryanodol ( 5 ) (Figure 2B).…”

“…A key discovery is that anhydroryanodol derivatives (and related C4-deoxy analogues) that possess C13–C18 unsaturation can be acylated directly to incorporate the crucial pyrrole-2-carboxylate ester. This represents a practical solution to one of the key challenges encountered in previous efforts to prepare ryanodine, 25 which should allow for the incorporation of an array of ester substituents at C3 for future SAR studies. In addition, we determined that a LiDBB-mediated reductive cyclization can be used to forge the final C1–C15 bond with the pyrrole ester intact.…”

“…The question of whether 4 can be converted to 1 was finally answered in recent studies by Inoue and co-workers . Having completed a 35 step synthesis of (±)-ryanodol in 2014, they subsequently determined that it can be advanced in 4 steps to an intermediate in which all but the C3 alcohol is protected.…”

“…The synthesis of ryanodine ( 1 ) commenced with tetracycle 12 , accessible in 11 steps from ( S ) - pulegone ( 11 ) and prepared in gram quantities on a single pass through the sequence (Scheme ). Prior studies have shown that the C4 and C12 alcohols, although tertiary, react faster with standard acylating reagents than the C3 alcohol. ,, Building off Inoue’s findings that cyclic boronates can be used to strategically protect the alcohols of ryanodol, the ring fusion diol of 12 was converted to the dioxaborinane by treatment with methylboronic acid. Subsequent LiBH 4 -mediated 1,2-reduction of 13 furnished the C3 alcohol with high diastereoselectivity.…”