How to add new environment without transferring formula to the next line?

Question

I'm trying to add environment align inside of enumerate, and this attempting leads to transferring formula which is not needed. How do I leave the formula on the place?

\item \begin{align*}  C\left(z\right) = \Biggl[
&  \prod\limits_{\ell=1}^{m} \frac{E_\ell\left(z_\ell\right)}{E_{\ell-
1}\left(z_\ell\right)}\Biggl] \! \Biggl[ \prod\limits_{\ell=1}^{m}
\frac{\prod_{u \in L_{z        _\ell}}\left(-u\right)^N\prod_{v \in
R_{z_\ell}} \left(v+1\right)^{L-N}}{\Delta\left(R_{z_\ell}; L_{z_\ell}
\right)} \Biggl] \\ 
&  \times \Biggl[\prod\limits_{\ell=2}^{m} \frac{z_{\ell-1}^L}{z_{\ell-1}^L-
z_\ell^L}\Biggl] \Biggl[\prod\limits_{\ell=2}^m\frac{\Delta\left(R_{z_\ell};
L_{z_{\ell-1}}\right)}{\prod_{u \in L_{z_{\ell-1}}}\left(-u\right)^N
\prod_{v \in R_{z_\ell}}\left(v+1\right)^{L-N}} \Biggl] 
\end{align*}
\end{enumerate}

Answer 1

Use aligned[t] in a simple $...$:

\documentclass{article}
\usepackage{amsmath}


\begin{document}

\begin{enumerate}
\item $ \begin{aligned}[t]\displaystyle C\left(z\right) = \Biggl[ & \prod\limits_{\ell=1}^{m} \frac{E_\ell\left(z_\ell\right)}{E_{\ell-1}\left(z_\ell\right)}\Biggl] ! \Biggl[ \prod\limits_{\ell=1}^{m}\frac{\prod_{u \in L_{z_\ell}}\left(-u\right)^N\prod_{v \in R_{z_\ell}} \left(v+1\right)^{L-N}}{\Delta\left(R_{z_\ell}; L_{z_\ell}\right)} \Biggl] \\
 & \times \Biggl[\prod\limits_{\ell=2}^{m} \frac{z_{\ell-1}^L}{z_{\ell-1}^L-z_\ell^L}\Biggl] \Biggl[\prod\limits_{\ell=2}^m\frac{\Delta\left(R_{z_\ell};L_{z_{\ell-1}}\right)}{\prod_{u \in L_{z_{\ell-1}}}\left(-u\right)^N\prod_{v \in R_{z_\ell}}\left(v+1\right)^{L-N}} \Biggl] \end{aligned} $
\end{enumerate}

\end{document} 

Answer 2

I use instead another simple solution using "two separate codes". For the item and for the first part of the formula, I have used $\displaystyle ......$; after I have used the classic ambient \align*. You can observe that for several and different items there are not any changes for the style of any long equations. There is also an advantage: = and \times have a vertical alignment as from screenshot.

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools,amssymb}
\begin{document}
\begin{enumerate}
\item  $\displaystyle C\left(z\right) = \Biggl[
\prod\limits_{\ell=1}^{m} \frac{E_\ell\left(z_\ell\right)}{E_{\ell-
1}\left(z_\ell\right)}\Biggl] \! \Biggl[ \prod\limits_{\ell=1}^{m}
\frac{\prod_{u \in L_{z_\ell}}\left(-u\right)^N\prod_{v \in
R_{z_\ell}} \left(v+1\right)^{L-N}}{\Delta\left(R_{z_\ell}; L_{z_\ell}
\right)} \Biggl]$
\begin{align*}
&  \times \Biggl[\prod\limits_{\ell=2}^{m} \frac{z_{\ell-1}^L}{z_{\ell-1}^L-
z_\ell^L}\Biggl] \Biggl[\prod\limits_{\ell=2}^m\frac{\Delta\left(R_{z_\ell};
L_{z_{\ell-1}}\right)}{\prod_{u \in L_{z_{\ell-1}}}\left(-u\right)^N
\prod_{v \in R_{z_\ell}}\left(v+1\right)^{L-N}} \Biggl] 
\end{align*}   
\end{enumerate}
\end{document}