As securities watchdogs crack down on complex investments that promise mom-and-pop investors access to strategies of trading pros, Wall Street is finding a way to sell the same products in places those regulators don’t reach.

The investments, which follow proprietary indexes developed by banks including JPMorgan Chase & Co. and Credit Suisse, are quietly spreading into more opaque retail markets. Once popular in structured notes, a form of bank debt bundled with derivatives, they are now helping pump up sales of certificates of deposit and insurance annuities. Specialized indexes now underpin more than a fifth of the $13.8 billion market for indexed annuities-investments that target retirees.

The shift moves oversight largely out of the purview of the U.S. Securities and Exchange Commission and into a patchwork of state and federal regulation. That may frustrate efforts to supervise a burgeoning array of products that regulators have questioned for their complexity, fees and actual returns.

While sales of proprietary index notes shrunk to $157.9 million in 2014, some Wall Street banks use the strategies in a significant amount of the structured certificates of deposit they sell, according to Dayna Kleinman, senior product manager for alternative investments at Robert W. Baird & Co. in Chicago.

Proprietary indexes can be useful, said Steffen Scheuble, chief executive of Solactive AG, which has created about 700 of them. “If you’ve got a rules-based strategy, it’s cheaper and more transparent” than sticking your money with an active manager, he said.

Banks that sell proprietary index notes include warnings of their risks in offering documents, such as that investors may not receive any returns.

The indexes became popular right after 2008, when the Standard & Poor’s 500 index plunged the most in seven decades, according to Vinit Srivastava, who manages strategy indexes at S&P Dow Jones indexes.